Showing papers by "University of Konstanz published in 1974"

Statistical analysis of alamethicin channels in black lipid membranes.

Abstract: Pore formation of alamethicin has been studied by the analysis of steadystate fluctuations of single-pore conductances. An aggregation model is proposed where transitions to the next higher or lower pore state occur by uptake or release of one monomer. It is assumed that alamethicin forms an elongated loop in the bilayer. The main voltage-dependent step is the insertion of this monomer into the membrane after complexation with a cation. This mechanism is equivalent to dipole orientation in an electric field. Pore formation is restricted by the energy required to enlarge the channel in the membrane.

Studies of Glutamate Dehydrogenase

Abstract: The association-dissociation equilibrium of beef-liver glutamate dehydrogenase was investigated in phosphate buffer of different ionic strengths and in the presence of the organic solvents ethylene glycol and dioxane. For some of the solvents the temperature dependence of the equilibrium constant was analyzed. The dependence of the apparent weight-average molecular weight on enzyme concentration shows always the same general shape as that found earlier in 0.067 M phosphate buffer at pH 7.6. Each curve can be fitted assuming an open association-dissociation equilibrium with one single equilibrium constant for all steps and with one single value for the second virial coefficient. Within the limit of the experimental error, the second virial coefficient is found to be the same for all systems (8–9 nmol × 1 × g-2), whereas the equilibrium constants vary by orders of magnitude. At low ionic strength a very small temperature dependence was found with a reduced association at higher temperatures. At high ionic strength a temperature maximum of the equilibrium constant is observed. Preferential phosphate binding is indicated by the high values of the refractive index increment. The organic solvents investigated here (ethylene glycol, dioxane) cause a dissociation into the unimers (oligomers). The equilibrium constant decreases from 9 × 105 M-1 in aqueous phosphate buffer to 5.2 × 103 M−1 in 6 M ethylene glycol and to 2.4 × 105 M−1 in 2 M ethylene glycol. The effects obtained with 5% dioxane (0.57 M) are similar to that with 2 M ethylene glycol. In both cases a sharp decrease of the association with increasing temperature is observed. The reaction enthalpy and the reaction entropy of the association reaction decrease with decreasing temperature in all solvents, the enthalpy being positive in the absence and negative in the presence of organic solvents. The free enthalpy of reaction, however, increases with increasing temperature in buffers of high and low ionic strengths but decreases in the presence of organic solvents. The association reaction at high ionic strength appears mainly to be entropy-driven whereas in the presence of the organic solvents the reaction seems to be energy-driven at all temperatures. It is suggested that the organic solvents interfere with the hydrophobic interactions between the unimers. The positive reaction enthalpy in the presence of high salt concentrations indicates that the attractive electrostatic forces are largely shielded, whereas shielding is imperfect for the repulsive forces which even seem to be enhanced. This could be due to a preferential binding of anions which would tend to increase the negative net charge and could explain the difference observed between the effects of phosphate and chloride solutions of identical ionic strength.

Electric organ discharge interaction during interspecific agonistic behaviour in freely swimming mormyrid fish

Abstract: A data acquisition technique is described, which uses a digital analyser to measure off-line the lengths of intervals between events on two (or more) lines. The method compensates for cumulative flutter of tape recorders; thus the temporal relationships between series of events on different lines are maintained.

Temporal progress of muscle adaptation to endurance training in hind limb muscles of young rats. A histochemical and morphometrical study.

Abstract: The soleus, rectus femoris and gastrocnemius muscles of young rats were studied after 3, 6 and 12 weeks of treadmill training. The muscle fibers were characterized histochemically by their succinate dehydrogenase (SDH) and myofibrillar ATPase activity, and morphometrically by their cross-sectional areas, which were corrected for different body weights of control and trained animals. After 12 weeks of training the mean area of fibers in the muscles studied was not significantly different from the controls, as expected. In the soleus muscle the percentage of the fast-twitch fibers was decreased as a result of their transformation into slow-twitch fibers. Trained soleus muscles were the only muscles showing pathologically altered fibers, suggesting overload. The percentages of fiber types and their areas exhibited changes specific for the muscles and muscle regions studied. From these results it is concluded that the adaptation follows the sequence “proportional adaptation of morphometrical parameters, disproportional adaptation of the areas of fiber types, and disproportional adaptation of the percentages and/or the areas of the fiber types”. It is shown by comparison with the literature that this sequence may be generalized to a sequence of increasing “expense” necessary for the adaptation to increasing stimuli, and that the most decisive factors for adaptation are work load, frequency of exercise, period of training, and the age of the subject at the initiation of the training.

Some functions involved in bacteriophage T7 genetic recombination

Abstract: Bacteriophage T7 genetic recombination is reduced in the absence of the viral coded endonuclease I, DNA polymerase and exonuclease molecules. The results also suggest that the T7 gene 4 product may be involved in recombination. The rec genotype of the host bacterium does not influence T7 recombination.

The effect ofHabrobracon venom on excitatory neuromuscular transmission in insects

Abstract: 1. The action of the venom of the braconid waspHabrobraconhebetor was studied in nerve-muscle preparations of its host the mealmoth larva (Ephestia kuhniella) and of the locust (Locusta migratoria) by comparing physiological properties of normal and paralysed preparations. 2. The venom neither had an effect on nervous conduction nor on membrane potential, current-voltage relationship, graded electrogenesis and contraction of the muscle fibres. 3. Inhibitory transmission was not blocked. Excitatory junction potentials were either greatly diminished or absent during paralysis. They could not be restored to normal size by increasing the external Ca concentration. 4. The postjunctional sensitivity to L-glutamic acid was not significantly altered by the venom. 5. In paralysed locust preparations miniature excitatory junction potentials were still observed, but their frequency was reduced to as little as 1% of controls. Their amplitude distributions were similar to controls, except that the proportion of very large mejps was somewhat higher. 6. In weakly paralysed preparations tetanic nerve stimulation caused facilitation and posttetanic potentiation of the reduced ejps. With more extensive paralysis, tetanic stimulation increased the frequency of the mejps and a few of the stimuli were followed by mejp-like ejps. 7. Raising the osmolarity of the saline increased the mejp-frequenoy of paralysed preparations significantly less than in control preparations. 8. The thiol oxidizing compound diamide caused a large increase of mejpfrequency in controls but completely blocked spontaneous release in paralysed preparations. These effects could be quickly reversed by a subsequent application of the disulfide reducing agent dithiothreitol. 9. It is unlikely that the purely presynaptic effect ofHabrobracon venom is on the electrical properties of the excitatory nerve terminals. It is discussed whether the release mechanism or the supply of transmitter are affected. There may be a specific affinity of the venom to glutaminergic synapses.

The influence of the wing sense organs on the flight motor pattern in maturing adult locusts

Abstract: Immediately after destruction of the sense organs in the wings and wing-hinges, the wing-beat frequency of matureLocusta migratoria decreases to 50–60% of the initial value. During the following days the frequency increases again. Similar results were obtained with young adult locusts. The same operation immediately after the last ecdysis does not prevent the appearenee of the normal flight pattern. The increase of the frequency during the first weeks of adult life is about the same in operated and non-operated locusts. Muscle recordings of old and young adults show similar changes of the flight motor pattern after destruction of the wing receptors. The results indicate that the development of the flight motor program is not dependent on wing afferents. The commands of the wing sense organs raise the frequency of the central flight oscillator by the same amount in both young and old adult locusts.

Electrical resistivity of amorphous alloys

Abstract: Amorphous alloys of Ga, Sn, Pb and Bi with Cu, Ag and Au are produced by evaporation on a cold substrate. The residual resistivity, the temperature dependence of the resistivity, the transition temperature of superconductivity and the temperature of the amorphous-crystalline transformation are measured. We observe e.g. that the residual resistivity increases with the noble metal concentration, and that the temperature coefficient of the resistivity of the Au alloys is always negative. In these two respects amorphous alloys differ in behaviour from the corresponding liquid alloys. These observations can be correlated with the atomic energy levels of the free atoms.

Nucleophilic Addition at the Photoexcited Flavin Cation: Synthesis and Properties of 6‐ and 9‐Hydroxy‐Flavocoenzyme Chromophores

Abstract: 1 The method of “photohydroxylation” of azaheteroaromatics, known in the phenazine and alloxazine series, has been applied to flavin (= isoalloxazine) derivatives FloxH. The products habe been identified as 6- and 9-hydroxyflavins (Flox-6/9-OH). On this basis it was possible to assign a 6-hydroxyflavin structure to the new flavocoenzyme chromophores described by Mayhew et al. in the accompanying paper. 2 In agreement with the data obtained by Dekker et al. in a recent laser study on alloxazine, we find that the reaction proceeds via addition of water, alcohols or carboxylic acids by the lowest excited singlet state of flavin with synchronous intramolecular hydrogen shift from the site of RO-addition [= C(6) or C(9)] towards N(5) and subsequent oxidation of the 6/9-hydroxy-1,5-dihydroflavin formed. 3 The photoaddition reaction occurs in competition with photodehydrogenation of available CH-bonds by the lowest excited flavin triplet, in which case the CH-substrate may be provided by the solvent ROH or, when R = H, by methyl groups of a second flavin molecule (= flavin autophotolysis). This photodehydrogenation may be suppressed by O2-dependent triplet quenching and/or by applying a positive charge to the flavin (RFIOxH+, R = H or alkyl) in order to inhibit the reaction with itself. 4 Differentiation of 6- and 9-hydroxyflavin isomers was possible by double resonance evaluation of the proton coupling between the residual Ar-H and the adjacent methyl group, by evaluation of the electron paramagnetic resonance hyperfine coupling for the residual Ar-H in the radical cations 1,5-RHFl-6/9-OH, and by formation of cupric chelates from 6-hydroxy-flavin as compared to the lack of metal affinity in the 9-isomer. 1H-Nuclear magnetic resonance, electron paramagnetic resonance and visible spectra and their dependence on the state of ionisation of hydroxyflavins are discussed. Hydroxyflavin anions as well as their N(1)-alkylated meso-ionic derivatives are shown to exhibit characteristic long-wavelength absorption bands ranging from 50 to 800 nm with maxima around 600 nm and low molar absorption coefficients between 500 and 3000 M−1cm−1 (as compared to ∼ 104 for the first π, π* transition at ∼ 400nm), which might be readily mistaken for “flavin-substrate charge transfer” bands. 5, 9-Methoxy-flavins could be spectroscopically related to 9:10-bridged cyclic hemiacetals obtained upon ring closure of 9-hydroxy groups with 10β-carbonyl functions. Such cyclic hemiacetals can be trapped in the acid photolysis of 10-(hydroxyalkyl)-flavins, e.g. riboflavin or flavocoenzymes, in a sequence of 10-side-chain dehydrogenation and N(1)-cycloaminal cation formation of the 10β-carbonyl formed, which results in a quenching of the further photolysis in favor of 9-photoaddition.

Thermodynamics of the Helix‐Coil Transition of (dG‐dC) Oligomers

Abstract: The dissociation of short double-helical DNA with the alternating sequence (dG-dC)n (dG-dC)n into the corresponding single strands was studied as a function of chain length, oligomer concentration and temperature in 10M NaCl, pH 72 by hypochromicity and equilibrium centrifugation The melting curves are analyzed according to an “all-or-none” model and the following thermodynamic parameters were deduced The change in free energy for formation of a dG · dC base pair adjacent to another one, ΔG0s, =−22 kcal/mol at 37°C; the corresponding change in enthaply, ΔH0s, =−111 ± 05kcal/mol and in entropy, ΔS0s, =−279 ± 1 cal · K−1· mol base pair−1 The stability constant, s, is 104 ± 5 at 25°C The formation of the first base pair from two single strands involves a nucleation parameter, β, = 16 10−4 M−1 at 25°C, which is similar to the corresponding values determined for different RNA oligomers The enthaply change associated with the formation of the first base pair, −ΔH0βs, =+3±5 kcal/mol which is very close to zero These thermodynamic parameters are compared with those for the dA · dT base pair At physiological conditions, it is estimated that the stability of one dG · dC pair is very close to that of two dAdT pairs The data imply that the opening of a unique sequence of DNA in a cell is a very improbable event

Studies on methylmalonyl-CoA mutase from Propionibacterium shermanii.

Abstract: 1 Methylmalonyl-CoA mutase from Propionibacterium shermanii has been purified according to a modification of the method described by Wood et al. in 1964. 2 The final mutase preparation was homogeneous in the ultracentrifuge showing the following hydrodynamic properties: 8020,W= 7.25 S, D020,w= 5.71 F. Its molecular weight was calculated to be 124000. A similar molecular weight was indicated also by gel-filtration on a calibrated Sephadex G-100 column both for the mutase apo-enzyme and the hydroxycob(III)alamin · mutase complex. 3 On treatment with guanidine hydrochloride methylmalonyl-CoA mutase dissociates into two polypeptide chains, which are similar, though not identical in size. Dodecylsulphate electrophoresis indicated for the molecular weights of the two polypeptide chains 61000 7plusmn; 3000 and 66000 ± 3000, respectively. 4 In the presence of excess hydroxycob(III)alamin methylmalonyl-CoA mutase was crystallised as pink needles consisting of the enzymically inactive inhibitor · protein complex.

Regulation of the Mammalian Pyruvate Dehydrogenase Multienzyme Complex by Mg2+ and the Adenine Nucleotide Pool

Abstract: The activity of the mammalian pyruvate dehydrogenase multienzyme complex in vitro depends on the composition of the adenine nucleotide pool (energy charge). An increase of the energy charge from 0.5 to 0.8 can cause a decrease of the activity of the complex by 70%. At high energy charge, i.e. when the amount of AMP and ADP in the adenine nucleotide pool is low, ATP inhibits the pyruvate-dehydrogenase phosphatase, the enzyme which reactivates the pyruvate dehydrogenase complex by dephosphorylation. ATP causes this effect by binding Mg2+ essential for phosphatase activity, because it binds Mg2+ much more strongly than ADP or AMP. From the three enzymes of the multienzyme complex requiring Mg2+ as cofactor, pyruvate dehydrogenase, pyruvate-dehydrogenase kinase, and pyruvate-dehydrogenase phosphatase, it is shown that only the latter has a Km for Mg2+ higher than the physiological magnesium concentration. Its Km is 20 mM in phosphate buffer. Therefore the variation of the concentration of unbound Mg2+ caused by a variation of the composition of the adenine nucleotide pool (energy charge) in vivo can be meaningful only for the pyruvate-dehydrogenase phosphatase activity. The phosphorylation-dephosphorylation cycle of the pyruvate dehydrogenase complex results in a net consumption of ATP unless this futil cycle is inhibited. At high ATP concentrations (energy charge > 0.5) this inhibition takes place by binding Mg2+ ions. Adenosine 3′: 5′-cyclic monophosphate has no effect on the activity of the pyruvate dehydrogenase complex. Controversial reports on this matter by other authors are discussed.

Nucleoside, XIV. Zur Synthese des 4‐Amino‐7‐oxo‐7,8‐dihydropteridin‐N‐8‐β‐D‐ribofuranosids — ein strukturanaloges Nucleosid des Adenosins

Abstract: Zur Darstellung verschieden substituierter 2- und 4-Amino-7-oxo-7,8-dihydropteridin-N-8-β-D-ribofuranoside wird eine neue Variante der „Silyl-Methode” beschrieben, die in der Lewis-Saure katalysierten Schmelzkondensation von 7-(Trimethylsiloxy)pteridinen 11 – 18 mit 1-O-Acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose (20) besteht. Die Reaktion verlauft unter sterischer Kontrolle und fuhrt zu β-Ribosiden 21 – 38, wobei die Anwesenheit von Elektronendonatorsubstituenten in 2-Stellung des Pteridinringes eine N-8-Substitution zusatzlich gunstig beeinflust. Zur Sicherstellung der Strukturen der nach chromatographischen Verfahren isolierten neuen Substanzen werden UV-, NMR- und CD-Spektren aufgenommen sowie pK-Werte bestimmt. Nucleosides, XIV. Synthesis of 4-Amino-7-oxo-7,8-dihydropteridine-N-8-β-D-ribofuranoside – A Structural Analogue of Adenosine The preparation of differently substituted 2- and 4-amino-7-oxo-7,8-dihydropteridine-N-8-β-D-ribofuranosides is described using a new variant of the „silyl method” which involves the Lewis-acid catalysed fusion of 7-(trimethylsiloxy)pteridines 11 – 18 with 1- O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose. The reaction proceeds under steric control and leads to the β-ribosides 21 – 38, the presence of electron donating substituents in the 2-position of the pteridine ring favourably influencing a N-8 substitution. U. v., n. m. r. and c. d. spectroscopic data as well as pK-values were used to elucidate the structures of the new substances, which were isolated by chromatographic techniques.

Photochemie des 10‐Phenylisoalloxazins: Intramolekulare Singulett‐ und intermolekulare Triplett‐Reaktionen

Abstract: 10-Phenylisoalloxazin (2a) reagiert im ersten angeregten Triplett-Zustand bei der Umsetzung mit verschiedenen Substraten analog dem Lumiflavin ( = 7,8,10-Trimethylisoalloxazin) unter Bildung des 1,5- Dihydroisoalloxazins bzw. 5- oder 4a-alkylierter Dihydro-Derivate. – Der erste angeregte Singulett-Zustand reagiert hingegen unter Photocyclisierung in einer protonen-katalysierten, Reaktion, die als intramolekulare, elektrophile Photosubstitution des N(1)-Atoms am Phenyl-Substituenten beschrieben werden kann. Dabei entsteht das heterocyclische System des 5H-Benzimidazo[1,2,3-ij]benzo[g]pteridin-6,8(7H)-dions (11), das auch durch Photocyclisierung des 1,3-Diphenylalloxazins (6) zuganglich ist. Der oxidative Abbau von 11 wird untersucht. The Photochemistry of 10-Phenylisoalloxazine: Intramolecular Singlet and Intermolecular Triplet Reactions From the first excited triplet state of 10- phenylisoalloxazine (2a) photoreduction or photoalkylation occurs in a way analogous to lumiflavin ( = 7,8,10-trimethylisoalloxazine) to yield 1,5-dihydroisoalloxazine or 5-and 4a-alkylated dihydro-derivatives thereof. – The first excited singlet state, however, undergoes photocyclization in a proton-catalyzed reaction, which can be described as an intramolecular, electrophilic photosubstitution of the N(1)-atom on the phenyl-substituent, yielding the new heterocyclic system of 5H-benzimidazo[1,2,3-ij]-benzo[g]pteridine-6,8(7H-dione (11), which is also formed on photocylization of 1,3-diphenylalloxazine (6). The oxidative degradation of 11 has been studied.

Pteridine, LXIV: Synthese und Eigenschaften von Thiolumazinen

Abstract: Einige 3-Methyl-2-thiolumazine (6–9) werden synthetisiert und durch pK-Werte sowie UV-Spektren charakterisiert. Die Umsetzungen der 2-Thiolumazine 11–13 mit 1,2-Dibromethan bzw. 1,3-Dibrompropan fuhren uberwiegend zu Thiazolo- bzw. Thiazino[2,3-b]pteridinen 15–19, wogegen die [3,2-a]-Isomeren nur als Nebenprodukte gebildet werden, wie im Falle des 7,8-Dimethyl-5-oxo-5H-1,2-dihydrothiazolo[3,2-a]pteridins (20) eindeutig nachgewiesen werden konnte. Auf die oxidativen Veranderungen verschiedener Thiolumazine durch Hydroperoxide wird hingewiesen. Pteridines, LXIV: Synthesis and Properties of Thiolumazines The synthesis of various 3-methyl-2-thiolumazines (6–9) is described and their characterization based on pK values and UV spectra. The reaction of 2-thiolumazines (11–13) with 1,2-dibromoethane or 1,3-dibromopropane leads preferentially to the formation of thiazolo- and thiazino[2,3-b]pteridines (15–19), whereas the [3,2-a]isomers are minor reaction products as seen from the isolation of 7,8-dimethyl-5-oxo-5H-1,2-dihydrothiazolo[3,2-a]pteridine (20). The oxidative transformations of various thiolumazines by hydroperoxides are mentioned.

Photochemie des (Iso) Alloxazins, III. Intramolekulare Photodealkylierung von 10-Alkylisoalloxazinen, eine Modellreaktion für den Riboflavin-Photoabbau

Abstract: Eine Anzahl 10-Alkyl-und 10-Cycloalkylisoalloxazine wird synthetisiert. Sowohl bei anaerober als auch aerober Bestrahlung werden in einem cyclischen Synchronmechanismus die Alkylgruppen, auser Methyl, Athyl und Neopentyl, als Alken bzw. Cycloalken abgespalten, wobei Alloxazin entsteht. Aerob verlauft diese Photodealkylierung uber den ersten angeregten Singulett-Zustand, anaerob uber Singulett- und Triplett-Zustand im Verhaltnis 1:8. Durch Vergleich der Reaktionsgeschwindigkeiten kann fur den Ubergangszustand ein ebener Sechsring der N(1)–C(10a)–N(10)–C(1′)–C(2′)– H(2′)-Gruppierung wahrscheinlich gemacht werden; die Quantenausbeuten sind proportional der Wahrscheinlichkeit, mit der diese Konformation eingenommen werden kann. – Der Photoabbau des Riboflavins wird mit diesen Ergebnissen verglichen. Photochemistry of (Iso)Alloxazines, III. Intramolecular Photodealkylation of 10-Alkylisoalloxazines, a Model Reaction for Riboflavine Photodegradation A number of 10-alkyl- and 10-cycloalkylisoalloxazines have been synthesized. On irradiation either anaerobically or aerobically the alkyl groups (except methyl, ethyl and neopentyl) are split off in a synchronous cyclic mechanism yielding alloxazine and an alkene or cycloalkene, resp. This photodealkylation proceeds aerobically via the first excited singlet state, anaerobically via singlet and triplet state in a ratio of 1:8. By comparison of the reaction rates a plane six-membered ring of the N(1)–C(10a)–N(10)–C(1′)–C(2′)– H(2′)-group can be shown to be the transition state; the quantum yields are correlated to the probability of occupying this state. – The results are compared to the photodegradation of riboflavine.

Growth and synchronization of cell division in the alga Bumilleriopsis filiformis

Abstract: 1. Axenic cultures of Bumilleriopsis were cultivated in mineral medium under continuous illumination and diluted every two days. The growth has been calculated to be 0.50 log10/day units at the optimum temperature of 23 to 24° C. 2. In liquid medium one cell normally divides into 2(n) daughter cells (=aplanospores). Under our culture conditions generally 8 and 16 daughter cells are produced by one Bumilleriopsis cell within 48 h. 3. Bumilleriopsis filiformis is the first Xanthophycean species to be synchronized. Light-dark changes induce some synchronization, but a high percentage of simultaneous division and rapid completion of a cell division burst is brought about most effectively by cycles composed of strong light/dim light/temperature changes (LS-T). With the most effective combination tested, i.e. 33 h of white 15000 lux light followed by 15 h of white 3000 lux (dim) light at temperatures of 24° in the first and 19° C in the second light phase (=33:15 h/15000:3000 lux/24:19° C), 96% of the cells divide synchronously. The burst occurs in such a manner that, on an average, 50% of the cells have divided 38 h after the start of the cycle. These synchronizing conditions were maintained up to 30 cycles without interruption (=continuous synchronization). 4. Cell wall formation is observed (by staining mother cells with methylene blue) 1 to 2 h before the cells divide completely and their daughter cells are released.

Reassociation Kinetics of Nuclear DNA from Physarum polycephalum

Abstract: Nuclear DNA of Physarum is made up of 45% repeated (c0t1/2= 0.07–0.7 mol · 1−1· s) and 55% non-repeated base sequences (c0t1/2= 500–1100 mol · 1−1· s) and has a complexity of 130 times the Escherichia coli genome. DNA replicated early in the S phase of the cell cycle, contains few repeated base sequences (below 10%).

Turnover of malate-dehydrogenase isozymes in rabbit liver and heart.

Abstract: 1. The cytosolic and mitochondrial isozymes of malate dehydrogenase were purified from rabbit heart. Antisera against the two isoenzymes were obtained from sheep. The antiserum against mitochondria] malate dehydrogenase showed no cross-reaction with the cytosolic isozyme and vice versa. 2. Apparent turnover rates of the two isozymes were determined in rabbit liver and heart by means of single pulse labelling with ['4C]leucine and isolation of the enzymes labelled in vivo by the immunochemical method. The apparent half-lives of the cytosolic and mitochondrial isozyme are 8.6 and 21.9 h in liver and 21.6 and 33.9 h in heart. These values are shorter than the apparent halflives of total soluble protein of liver and heart. 3. Turnover of mitochondrial malate dehydrogenase in rabbit liver was also calculated from the kinetics of enzyme accumulation and regression during a starvation-refeeding cycle. The half-life during starvation is 15.4 h and in the refeeding period 19.6 h. The approximately two-fold increase in mitochondrial malate dehydrogenase content of liver during starvation appears thus to be brought about by an increased rate of synthesis. The occurrence of two major separable forms of malate dehydrogenase in most animal tissues is well established [l - 51. One form of the enzyme is found in the mitochondria, the other is present in the cytosol. These two types reveal different catalytic, physical and immunological properties and appear to be controlled by separate genes [6- 101. In a preceding paper we have presented direct evidence for the biosynthesis of both isozymes at the cytosolic ribosomes of rabbit liver in vivo[11]. The present paper reports data about the turnover of malate dehydrogenase isozymes in rabbit liver and heart. After single pulse labelling with ['4C]leucine the two isozymes were isolated from the tissue homogenates by immunopi-ecipitation with specific antibodies prepared against the isozymes purified from rabbit heart. In addition, the turnover of mitochondrial malate dehydrogenase was determined during a starvationrefeeding cycle.