Showing papers in "Biochimie in 1975"

Kinetic amplification of enzyme discrimination.

Abstract: Summary The dependance of the accuracy of enzymatic systems on the mechanism of the catalyzed reaction is investigated, using a probabilistic approach. Certain mechanisms of reaction, involving a delay in one of the steps act as kinetic amplifiers of molecular discriminations. The relationship between our scheme for a delayed reaction [1] and Hopfield's scheme [2] is discussed.

Characterization of differentiated and dedifferentiated clones from a rat hepatoma.

Abstract: Summary An analysis of clonal variability of derivatives of the rat hepatoma line H4IIEC3 has shown that the overwhelming majority of clones express in a stable fashion a number of liver specific functions, including secretion of serum albumin, activity of the liver specific isozymes of alcohol dehydrogenase (EC1.1.1.1) and aldolase (EC4.1.2.13), and high basal activity and hormone inducibility of tyrosine aminotransferase (EC2.6.1.5) and alanine aminotransferase (EC2.6.1.2). The differences in level of expression of these functions cover a range of five to ten-fold, and the variations do not appear coordinated within or between clones. Seven clones, which differ from the above ones both in morphology and in the expression of liver specific functions, have been isolated. In five of them, no expression of any of the functions is detectable, while two of them show diminished but significant expression of two or three of the functions. In addition, an unexplained negative correlation between activity of glucose-6-phosphate dehydrogenase (EC1.1.1.49) and the expression of liver specific functions is described.

Pleiotropic mutations affecting sporulation conditions and the syntheses of extracellular enzymes in Bacillus subtilis 168.

Abstract: Summary Pleiotropic mutations of the chromosome of Bacillus subtilis 168 affecting simultaneously the levels of extracellular levansucrase and proteolytic activities are described. These mutations have been mapped at the sacU locus identified by PBS 1 mediated transduction. Several pleotropic hyperproducers and pleiotropic hypoproducers of these extracellular enzymatic activities, genotypically designated sacU h and sacU − respectively, have been isolated. sacU h mutants are capable of sporulation in rich media or in mineral media containing amino acids in the presence of an excess of glucose in both cases; under these conditions the sporulation of the wild type strain 168 is inhibited. One pleiotropic mutation conferring hyperproduction of levansucrase and proteolytic activities was mapped at the sacQ locus distant from sacU . The sacU and sacQ mutants may be affected in a not yet identified regulation mechanism which controls simultaneously the production of several extracellular enzymatic activities and the sporulation conditions of B. subtilis 168.

Enzymatic and non-enzymatic assay of superoxide dismutase.

Abstract: Summary Superoxide dismutase from breef brain and rat liver was assayed in an enzymatic system, using xanthine oxidase, and a non-enzymatic system, based on aerobic reduction of nitro-blue tetrazolium in presence of phenazine methosulphate. The non-enzymatic assay is rapid and simple and permits simultaneous analysis of many samples. Similar results are found by the two methods of assay of superoxide dismutase.

Isolation of mutations in Dictyostelium discoideum affecting α-mannosidase

Abstract: Summary We have isolated mutant strains of the cellular slime mold, Dictyostelium discoideum , which specifically lack α-mannosidase-1. This enzyme accumulates during the developmental phase of the life cycle. A minor isozyme, α-mannosidase-2, was recognized which accumulates only during culmination and makes up about 10 p. cent of the total activity in fruiting bodies. The minor isozyme has a pH response distinctly different from the major isozyme. Loss of the major isozyme, α-mannosidase-1, does not disrupt the normal sequence of biochemical differentiations.

Lysogenization by bacteriophage lambda. III. Multiplicity dependent phenomena occuring upon infection by lambda.

Abstract: Summary Lysogenization by bacteriophage λ involves at least two multiplicity dependent processes [2, 3]. For the purpose of comparison, other multiplicity dependent phenomena which occur upon infection by λ have been reviewed. These include the inhibition of host'syntheses as already described by others [9] and two phenomena which are shown to be multiplicity dependent, host killing by phage unable to replicate and inhibition of cell division. It is also demonstrated that, in at least two cases (lysogenization by phage able to replicate and killing by phage unable to replicate) the multiplicity dependent character disappears at slow cellular growth rates. The significance of these results is discussed with regard to three models which are susceptible to account for multiplicity dependent phenomena in general.

Physical mapping of the att-N region of coliphage lambda: Apparent oversaturation of coding capacity in the gam-ral segment

Abstract: Summary Employing a series of λbio deletion mutants, the genes int, xis, Ea22, exo, bet, gam, kil, cIII, Ea10, ral and N and their known protein products have been ordered in respect to the physical map of λ, as constructed by electron microscopy of heteroduplexes between the λbio's and λattR-attL's (λatt2imm21 or λatt2imm434). Throughout most of this region the coding capacity is about saturated, as based on the known sizes of the λ proteins and the positions of the genetic and physical endpoints of the bio deletions. One baffling exception is the gam-kil-cIII segment, which cannot accommodate all three of the corresponding proteins. The second exception is the region between genes ral and N, which could code for a protein about 25,000 daltons, but for which no proteins have been identified. The space available for gene N suggests that its product must be a protein of no more than 15,000 daltons in size. Physical mapping of the bio386 deletion has established that proteins Ea42(int), Ea32(xis ?) and Ea22 are encoded in that order.

Pig heart mitochondrial ATPase : properties of purified and membrane-bound enzyme: Effects of flavonoids

Abstract: Summary Soluble ATPase (F1)has been purified from pig heart mitochondria. The purified enzyme had a high specific activity and was homogeneous as checked by ultracentrifugation and electrofocusing. It could be dissociated into subunits by cold-treatment or sodium dodecyl sulfate denaturation. The molecular weights of the two major and three minor subunits could be estimated by sodium dodecyl sulfate gel electrophoresis. The native enzyme had an isoelectric point of 5.2 while the cold-denatured enzyme showed three main bands focusing at pH 5.0, 5.2 and 5.4. Kinetic properties (Vm and Km (ATP)) have been compared for the soluble and membrane bound ATPase in presence of various anions. Inhibitory effects of Quercetin and other flavonoids have been tested in order to get an insight on the interaction between ATPase and its natural inhibitor.

The determination of the primary structure of the 16S ribosomal RNA of Escherichia coli. III. Further studies.

Abstract: Summary In this paper, we describe in detail the recent progress in the nucleotide sequence analysis of the 16S ribosomal RNA from E coli The sequence which has been partially or completely determined so far encompasses 1520 nucleotides, ie about 95 percent of the molecule Possible features of the secondary structure are suggested on the basis of the nucleotide sequence and data on sequence heterogeneities, repetitions and the location of modified nucleotides are presented

Post-translational modifications of human glucose-6-phosphate dehydrogenase.

Abstract: Summary Immunological, kinetic and electrophoretic (electrofocusing) properties of glucose-6-phosphate dehydrogenase have been studied in various hemopoietic tissues. The influence of molecular aging on these properties has been followed. o 1. The molecular specific activity defined as the ratio : enzymatic activity/immunological reactivity decreased during aging of cells which are no more capable of protein synthesis (platelets, red blood cells). 2. Each type of cells (polymorphonuclear, lymphocytes, platelets, red cells) shows a constant and particular pattern upon electrofocusing. Aging of erythrocytes is accompanied by the appearance of active bands with a lower isoelectric point. 3. In vitro incubation leads to the appearance of forms with a lower isoelectric point. This phenomene requires the presence of cellular factors in the tissue. It is not prevented by the addition of inhibitors of proteolysis. 4. The electrofocusing pattern is independent from the saturation in NADP+, the presence of reducing or oxidizing agents, the protein concentration. 5. The platelet enzyme shows several peculiarities, e.g. increased Km glucose-6-phosphate, abnormal pH curve, and a smaller effect of in vitro incubation on the electrophoretic pattern. 6. A connection between kinetic, immunological and electrophoretic properties is demonstrated for most cells. The forms with the lowest isoelectric point have also the lowest molecular specific activity and the highest Km for glucose-6-phosphate. Finally, the nature and meaning of postranslational alteration of glucose-6-phosphate dehydrogenase are discussed.

Long term cell culture of rat liver epithelial cells retaining some hepatic functions

Abstract: Summary Extensive studies of parameters conditioning high plating efficiency of epithelial liver cells at primary seeding allowed us to set up a technique for the routine culture of liver cells from rats of various ages (18 day-old pc to 7 month-old) in Ham F10 medium supplemented with 10 p. cent fetal calf serum and 10 p. cent human serum. Cultures, after several passages, or sometimes at primary seeding were free of fibroblasts. Cell lines were kept for over one year with a subculture splitting ratio of 1 to 10 each week. The generation time of the cells was 16–18 hours. Caryotype analysis showed a high majority of normal diploid and tetraploid cells. Various enzymes and metabolic pathways have been studied in primary culture and in cell lines: enzymes of the anaerobic metabolism of hexoses and metabolism of steroid hormones. Activity glucose-6-phosphatase was nearly lost in all cultures. Aldolase showed a specific liver activity with a cleavage ratio of phosphofructoses (F-1,6-diP/-1-P) equal to 1 or about 1 in several primary cultures and cell lines. Many metabolites arising from incubation of cell lines with 14C-labelled corticosterone, corticosterone-21-sulfate, testosterone and progesterone have been isolated and quantitated by gas liquid chromatography (GC) and mass fragmentography coupled to GC, using 14C/12C isotope ratio measurements. These metabolites indicate the presence in cultured cells of 3α/β-steroid-reductases, 4-enesteroid-reductases, 17β-steroid-oxido-reductase, 11β-steroid-oxido-reductase and ring hydroxylases.

Evidence for three enzymatic activities in one electrophoretic band of 3-phosphoglycerate mutase from red cells

Abstract: Summary Electrophoresis of 3-phosphoglycerate mutase from erythrocytes of man and several animal species has been performed on cellulose acetate strips In most cases the electrophoretic pattern of this enzymatic activity shows three bands 2,3-diphosphoglycerate phosphatase and diphosphoglycerate mutase from erythrocytes of the same species have been revealed after migration during the same electrophoresis We found that the band of 2,3-diphosphoglycerate phosphatase and the band of diphosphoglycerate mutase activities migrate at the same level as one of the bands corresponding to 3-phosphoglycerate mutase Here, we discuss the possible existence of a single molecule carrying three enzymatic activities

Caractérisation et propriétés physico-chimiques de la transcortine humaine

Abstract: Summary The molecular weight of human transcortin, calculated from the sedimentation coefficient, was found to be 49,500, thus slightly lower than previously reported values. After purification, human transcortin trended to polymerize rapidly, with participation of both non covalent bonds and one disulfide bridge per dimer. The physicochemical parameters, the amino-acid and carbohydrate composition were determined ; its stability was studied under different conditions. Preliminary structural studies showed that the N-terminal sequence of the polypeptide chain was : Met-Asp-Pro-Asn-Ala-Ala-Tyr-Val and that the C-terminal aminoacid was leucine.

Applications of Raman spectroscopy to biological macromolecules

Abstract: The Raman spectra of biological macromolecules arise from molecular vibrations of either the backbone chains or the side chains. The frequencies of the Raman bands lie in a region between 200 cm-1 and 3000 cm-1. From certain frequencies of the vibrations of the backbone chains one can determine the conformation or secondary structure of a macromolecule. Thus for polypeptides and proteins the frequencies of the Amide I and Amide III vibrations allow one to determine the averge conformation of their backbone chain. In polynucleotides and nucleic acids, the frequency of the phosphate diester stretch of the phosphate furanose chain varies between 814 cm-1 for A conformation and 790 cm-1 for B conformation. Raman spectra of the bases in nucleic acids can be used to determine base stacking and hydrogen bonding interactions. Thus Raman spectroscopy is an important tool for determining the conformation structure of proteins and nucleic acids.

Propriétés d'une ATPase Ca2+ ou Mg2+ dépendante des membranes plasmiques de lymphocytes

Abstract: Summary The Ca2+ - activated ATPase properties of isolated plasma membrane of small lymphocytes from young pig mesenteric nodes are characterised. This enzyme is activated either by Mg2+ or Ca2+. A (Na+ K+) activated ATPase can be measured if membranes are previously treated by Na deoxycholate. Concanavaline A, a lectin which induces transformation of lymphocytes, stimulates Ca2+ or Mg2+ - activated ATPases but has no effect on the (Na+ K+) - ATPase. The properties of the Ca2+ stimulated ATPase suggest either that it is involved in Ca2+ transport through the membrane or that it is bound to a contractile protein involved in membrane deformability and permeability.

The sugar part of x-caseins from cow milk and colostrum and its microheterogeneity+

Abstract: Summary Cow x -casein contains only three different sugars (Gal, GalNAc, NeuNAc). However detailed analyses achieved mainly by gas liquid chromatography suggested a microheterogeneity at the sugar level. After alkaline borohydride treatment, filtration on Bio-Gel P4 and paper chromatography, different carbohydrate parts were obtained. The two main compounds had the following molar compositions : GalNAc (1), Gal(1) and NeuNAc (1) and GalNAc (1), Gal (1) and NeuNAc(2). From these data and our previous sequence studies, some formulae of the polysaccharide part were proposed. One of them was closely related to the sugar equence of a glycopeptide with MN activity which was in agreement with our observation concerning a cross antigenic reactivity between the N blood group substances and the caseinoglycopeptides. All the polysaccharide parts isolated from colostrum caseinoglycopeptide were much more complex than those obtained from the normal glycopeptide, confirming an evolution of the sugar part as a function of time after parturition.

Presence of the methylester of 5-carboxymethyl uridine in the wobble position of the anticodon of tRNAArgIII from brewer's yeast

Abstract: The methylester of 5-carboxymethyluridine (mcm5U), its degradation product 5-carboxymethyluridine (cm5U) and the corresponding nucleotide (cm5Up) were isolated from brewer's yeast tRNAIII Arg or from the dodecanucleotide containing the anticodon. Their chromatographic and electrophoretic properties and their UV absorbing spectra were identical to that of the corresponding synthetic compounds. The gas chromatographic behavior and the mass spectrum of mcm5U obtained from tRNAIII Arg and of a synthetic sample were also identical ; the rare occurence of a thermal reciprocal bimolecular methyl-hydrogen transfer in the mass spectrometer ion source was observed. A mild alkaline treatment of tRNAIII Arg leads to the saponification of mcm5U into cm5U (within the tRNA), which can be again esterified in the presence of a yeast homogenate and (methyl-14C) S adenosylmethionine. The radioactivity was found in the mcm5U located in the wobble position of the anticodon of tRNAIII Arg. The presence of this odd nucleotide in that position could possibly restrict the codon-anticodon interaction of tRNAIII Arg.

Ovine pancreatic lipase : purification and some properties.

Abstract: Summary Lipase has been isolated from sheep pancreas. The lipoprotein complex formed in pancreas homogenates by the enzyme and endogenous lipids is split by treatment with acetone. Lipase is further purified by ion-exchange chromatography and gel filtration. The molecular weight and the amino-acid composition of ovine lipase are very similar to that of the porcine and bovine enzymes. As previously found in bovine lipase, no carbohydrate is covalently bound to the polypeptide chain which has a N-terminal residue of lysine. The study of the catalytic properties of ovine pancreatic lipase indicates that the enzyme is fully activated by colipase from various species in the presence of conjugated bile salt micellar solutions.

Modification of pyruvate kinase isozymes in prolonged primary cultures of adult rat hepatocytes.

Abstract: Pyruvate kinase isozymic changes were studied in the adult hepatocyte cultures, by electrophoretic, kinetic and immunological methods. We were able to maintain parenchymal cells from normal adult rat liver in non-proliferating monolayer cultures up to 10 days. Hepatocytes appeared to contain a dominant PK I type up to 4-5 days of culture. After day 5, PK III type was regularly present with PK I and after 7 days PK III type was always the only isozyme detected in culture. It must be pointed out that, by the Ouchterlony method and sometimes by electrophoresis, concentrated extracts from freshly isolated hepatocytes or starting hepatocyte cultures did also contain Pyruvate kinase PK III type. These results suggest that Pyruvate kinase III is present but partly repressed in the adult parenchymal cells and becomes derepressed in culture.

Analyse formelle de circuits de régulation génétique: le contrôle de l’immunité chez les bactériophages lambdoïdes

Abstract: Summary This paper is an attempt to describe and analyze in formal terms a genetic circuit which is rather complex and reasonably well disentangled: the control of immunity in lambdoid bacteriophages. Known models are expressed as logic equations, which relate the stade of activity of genes to variables of three kinds: genetic variables which describe the genotype of the organism, environmental variables like temperature and memorization variables. The value of each memorization variable (presence or absence of a gene product) is related to the value of the corresponding function (operation or not of the gene) by two characteristic time delays, an «establishment delay and a «decay delay. From the equations, one can derive matrices which facilitate comparison between models by showing which stable states are predicted by each model. Implications of current models, which had apparently remained cryptic, have been realized and experimentally tested. From the matrices, one can derive graphs which show the pathways (sequences of states) consistent with each model. These graphs are frequently branched and in these cases one has to analyze which conditions determine that one pathway rather than another one, is followed.

Lysogenization by bacteriophage lambda: II. - Identification of genes involved in the multiplicity dependent processes

Abstract: Summary We previously showed that, under conditions of rapid exponential growth, lysogenization of E. coli cells by phage λ requires that the cell is infected by at least 2 phages able to replicate their DNA, or 3 or 4 phages unable to replicate their DNA [ref. 4]. Since genes dealing with prophage integration appear not to be involved in these multiplicity dependent processes, a determination was made as to whether more than one copy of the genes involved in repressor synthesis or its activation are needed for lysogenization. The complementation patterns which we obtained indicate multiplicity effects involving gene cII (and, perhaps, cIII) in lysogenization by both phage able or unable to replicate. In the former case, we propose that cII protein (and, perhaps, cIII) both induces repressor synthesis and inhibits phage DNA replication. In lysogenization by phage unable to replicate, the data suggest that the expression of early phage genes and repressor synthesis in the course of lysogenization are mutually exclusive processes which do not take place on the same phage chromosome.

[The proteolytic system of Penicillium roqueforti. III. - Purification, properties and specificity of a protease inhibited by E.D.T.A].

Abstract: Summary An extracellular protease of Penicillium roqueforti , differing from the acid protease, has been isolated from culture media by ammonium precipitation, gel filtration and CM-cellulose chromatography. The purified preparation was homogenous on polyacrylamide gel electrophoresis at pH 9.0, 9.2 and 4.3. The molecular weight was estimated to be 20 000 daltons by gel filtration. Optimum pH for casein and hemoglobin digestion was respectively 5.5 and 4.2. The enzyme was thermostable (residual activity: 25 p. cent by the treatment at 100°C for 20 min pH 6.0) but it was very unstable at 65°C. Cobalt ions promoted an activation of 100 p. cent on casein hydrolysis. Chelating agents inhibited the enzyme activity completely. D.F.P., sulfhydryl reagents, D.A.N. had no effect on the activity. The protease II displays a specificity which is different from that found for neutral proteases sensitive to metal chelators, Disubstituted peptides which are classical substrates for these enzymes, (Z-X-Y-NH 2 , Y = Leu or Phe), are not split by the protease II. Two peptide bonds (Ala 14 -Leu 15 , Tyr 16 -Leu 17 ) are hydrolysed at a good rate on the oxydized insulin B-chain. The others peptide bonds usually hydrolysed by metalloproteases (His 5 -Leu 6 , His 10 -Leu 11 , Gly 23 -Phe 24 , Phe 24 -Phe 25 ) are not attacked by protease II.

Nucleotide sequences of the T1 and pancreatic ribonuclease digestion products from some large fragments of the 23S RNA of Escherichia coli

Abstract: Summary When the 23S RNA from E. Coli was pretreated for 1 h at 60° in the presence of Mg ++ and K + and then subjected to T 1 ribonuclease attack, resistant fragments were recovered from 3 regions of the molecule : region A (containing 470–500 nucleotides) located at the 5′ end of 23S RNA, region B (containing 520–550 nucleotides) located at the 3′ end and region C (containing 110–120 nucleotides) lying between region A and region B. The nucleotide sequences of the T 1 and pancreatic ribonuclease digestion products from these 3 regions have been studied and in most cases determined. In the course of these studies, a certain number of abnormal nucleotides, which are not methylated, have been encountered. A low level of sequence heterogeneity was detected.

Host interference with expression of the lambda N gene product.

Abstract: Summary We have searched among E. coli M72 (D, bio11cI857H1) temperature resistant survivors and have found two bacterial mutants, gro100 and gro101 which block λiλ and λi434 phage development but allow growth of their N-independent derivatives λiλ nin and λi434nin. It is not known yet whether these two mutants interfere with the production of the N gene product or with its function. At least part of the gro genotype maps at 12′ of the E. coli genetic map and is co-transductible by Pl with the lac locus.

Métabolisme comparé des phospholipides des organes effecteurs de l'osmorégulation chez l'anguille européenne (Anguilla anguilla)

Abstract: Summary The phospholipid composition from various organs of the fresh water eel, such as gill, kidney, gut, liver and muscle, were determined by thin-layer chromatography. The major phosphatides found in these tissues were PC, PE and SPH and minor constituents PS, PI, DPG, AP and also LPC in the gut. A greater percentage of PS and SPH occurs in the osmoregulatory effector organs such as gill, kidney, and gut. From in vivo comparative kinetic studies of the 32 P incorporation into the phospholipids, between 6 and 48 hours, certain remarkable features of phospholipid metabolism have been found in these tissues. A low uptake of inorganic 32 P into the tissue lipid phosphorus was observed in the eel at 15°C. The specific activity of the lipid phosphorus increased continuously in all tissues during 48 hours after 32 P injection. During this experimental period, phosphatidic acid and phosphatidyl inositol fractions were labelled most rapidly in gill, kidney and gut, while the specific activity of the phosphatidyl choline fraction remained low in these organs. In liver, the rate of PC formation appears to be faster than the PI and PE biosynthesis. In gill and gut, the PE showed greater turnover than the PC as measured by 32 P incorporation. In the eel, an euryhalin fish, the DPG of osmoregulatory effector organs has a high specific activity at all times. PS showed only a high specific activity in the gill. Labelling of SPH occured slowly in the various tissues, only becoming evident after 24 hours. The results are compared with those published for other poikilotherm and homeotherm vertebrates. Relative differences between the turnover of various tissue phosphatides are discussed with of reference to the general scheme on phospholipid biosynthesis and to the physiological functions of the various organs.

Levansucrase of Bacillus subtilis : reexamination of some physical and chemical properties.

Abstract: To give some support to researchs presently in progress in this institute, on the sequence elucidation and the X-Ray pattern of the levansucrase of B. subtilis, some physical and chemical properties of this enzyme were carefully reexaminated. The results explicit and on some points rectify previous reports from this laboratory. The molecular weight was measured by three different methods: sedimentation equilibrium, SDS-gel electrophoresis, gel filtration. They give an average value of 54000 g. From this molecular weight and the value of the Stokes' radius, an estimate of the frictional ratio f/fo was calculated. These results provide some knowledge about the size and the shape of the molecule. They are consistent with the electronic microscopy observations obtained elsewhere. The amino acid composition was determined from the acid hydrolysate. The nature of the sulfur containing aminoacid was established by analysis of (35-S)-labelled levansucrase : neither cysteine nor cystine were found in the molecule. The methionine residues appear essentially under unoxidized form. One terminal residue was characterized by the dansylation method using (14-C)-labelled dansyl chloride. An explanation of the affinity of the levansucrase on hydroxyapatite was attempted.

Enzymatic properties of prostaglandin synthetase from bovine seminal vesicles.

Abstract: The relative quantities of PGE2 and PGF2alpha biosynthesized are strictly dependent on the concentration of the precursor, arachidonate, on the nature and on the quantity of sulfhydrylated cofactor present The sigmoid aspect of the curve of PGF2alpha biosynthesis as a function of arachidonate concentration is directly dependent on the GSH concentration Treatment of the microsomes by low concentrations of HgCl (5 muM) modifies the kinetics of PGE2 and PGF2alpha biosynthesis Similarly, the presence of L-8027, a competitive inhibitor, alters qualitatively and quantitatively the production of PGE2 and PGF2alpha as a function of arachidonate concentration

Ternary solvents to investigate proteins at zub-zero temperatures.

Abstract: Summary Mixtures of water, ethylene glycol and methanol in different volume ratios have been selected to carry out kinetics of enzyme reactions at sub-zero temperatures with the intention to reduce maximally the viscosity. Density, viscosity and dielectric constant values of these mixtures as a function of temperature are reported, as well as values of the protonic activity of several buffers under such conditions. A procedure to avoid or delay the eventual damaging effect of methanol on proteins is described.

The properties of a uterine œstradiol-receptor after limited proteolysis(*)

Abstract: Summary The uterine oestradiol receptor exhibits multiple molecular forms depending upon experimental conditions. Mild proteolysis with trypsin transforms the receptor into a molecule of 60,000 daltons. No other molecular forms are formed. Neither the physicochemical parameters of the receptor-oestradiol interaction nor its specificity towards steroid hormones is affected. The altered molecule is relatively resistant against further proteolysis and the transformation is irreversible. The effect of trypsin is not specific; mild treatment with chymotrypsin, pronase and with an undefined endogenous protease results in the same structural change. Results will be discussed in the light of some recent observations in our laboratory on the interaction of the receptor with chromatin and on the subunit structure of the receptor, and will be compared with the findings of other authors concerning the effect of the « Receptor Transforming Factor .