Showing papers by "University of Dundee published in 1983"

Adaptation of unicellular algae to irradiance: an analysis of strategies

Abstract: SUMMARY 157INTRODUCTION 158AN.^LYTICAL METHODS 159LIGHT HARVESTING BY MICROALGAE 160RANGE OF PHOTON FLUX DENSITIES ALLOWING GROWTH AND PHOTOSYNTHESIS INPHOTOTHOPHIC MICROALGAE (GENOTYPIC ADAPTATION) 163 Growth 163Photosynthesis 165Photoinhibition 165 PHENOTYPIC ADAPTATION 168 Changes in amounts of pigments 168Interpretation of the effects of pigment changes: models 169Observed changes in P vs I curves 170 ENERGETIC CONSIDERATIONS 174 General 174Reduction of capital costs 175Reduction of maintenance costs 175Energetic costs of changing the photosyothetic apparatus 177S2-S3 decay 177Proton leakage due to passive uniport 178 PHYLOGENETIC ASPECTS OF DIFFERENCES IN LIGHT RESPONSES OF MICROALGALPHOTOSYNTHESIS AND PHOTOLITHOTROPHIC GROWTH 178 Phylogenetic diflerences in photosynthetic structures 178Comparison of tbe photosynthetic characteristics of green algae and otherpbototrophs 180 ECOLOGICAL CONSIDERATIONS 182ACKNOWLEDGEMENTS 185REFERENCES 185 SUMMARY Analysis of data in the literature relating to micrcalgal adaptations to different photon fluxdensities indicates that different algal classes have significantly different ligbt requirennents forgrowth and photosynthesis. Although there is some variability within each class, dinoflagellatesand blue-green algae generally photosynthesize and grow best at low photon flux densities.Diatoms also tend to be able to grow at very low photon flux densities (growth for some specieshas been reported at less than 1 fi.E m"' s~'). Comparison of the photon flux densities at whichphotoinhibition occurs in dinoflagellates and diatoms suggests that the former often experiencephotoinhibition at comparatively low irradiances. In contrast, diatoms often can toleraterelatively high light environments. This tolerance of a large absolute range of photon fluxdensities may, in part, explain why diatoms are often associated with spring blooms. Green algae* New address; Department of Botany, La Trobe University, Bundoora, Victoria, 3083 Australia.0028-646X/83/020157 + 35 S03.00/0 © 1983 The New Phytologist

The transport and function of silicon in plants.

Abstract: Summary A number of lines of evidence (Mr, number of -OH groups, measured fluxes at inner mitochondrial membranes) suggest the intrinsic PSi(OH)4 of about 10-10 m s-1 in the plant cell plasmalemma. While relatively low, such a PSi(OH)4 could maintain the intracellular concentration of Si(OH)4 equal to that in the medium for a phytoplankton cell of 5 μm radius growing with a generation time of 24 h. Such passive entry could not account for SiO, precipitation such as is required for scale (Chrysophyceae) or wall (Bacillariophyceae) production in terms of either the generation of a super-saturated solution or the quantity of SiO2 required; active transport occurs at the plasmalemma (and possibly at an internal membrane) of such cells. The energy required for silicification, even in a diatom with an Si/C ratio of 0.25, is only some 2% of the total energy (as NADPH and ATP) needed for growth; the energy cost of leakage of Si(OH)4 due to the intrinsic permeability of lipid bilayers to Si(OH)4 is never more than 10% of the cost of silicification. In vascular land plants the entry of Si from the soil into the xylem can involve a flux ratio (mol Si/m3 water) that is less than (e.g. Leguminoseae) equal to (e.g. many Gramineae) or greater than (e.g. Oryza, Equisetum) the concentration (mol m-3) in the bathing solution. Even the low influx of the Leguminoseae cannot be accounted for by the ‘lipid solution’ value of PS(OH)4, but requires entry coupled (phenomenologically) to water influx with a reflexion coefficient of about 0.9. The situation in most Gramineae is described by such a coupling with a reflexion coefficient near O, while the accumulation of Si (relative to water) in Oryza and Equisetum involves an apparent reflexion coefficient which is negative, i.e. an active transport system stoichiometrically related to water flux. Even in Leguminoseae with a transpiration-stream concentration of Si(OH)4 of only 20 mmol m-3 (cf. the soil solution at 200 mmol m-3), the fact that only I % of the water in the xylem is retained in the plant means that Si(OH)4 at transpirational termini approaches saturation; super-saturation, and precipitation of SiO, occurs in Gramineae and Equisetum. SiO2 precipitation occurs mainly near transpirational termini but can also occur in the xylem vessels and endodermis of roots, for example. Si(OH)2 mobility in the phloem seems to be very restricted. The energy costs of SiO2 relative to organic compounds as structural and defensive materials are in the ratio of 1:10-1:20 (on the basis of weight of material). The relative rarity of SiO2 as a structural material is discussed in the context of the evolution of Si(OH)4-transport mechanisms.

Brown-adipose-tissue mitochondria: the regulation of the 32 000-Mr uncoupling protein by fatty acids and purine nucleotides

Abstract: The increased proton permeability induced by the addition of a synthetic proton translocator to non-respiring hamster brown-fat mitochondria is unaffected by purine nucleotide addition. In contrast the permeability induced by fatty acids is inhibited by nucleotide, indicating that fatty acids act at the 32000-Mr uncoupling protein. Fatty acids lower the affinity of nucleotide binding to the 32000-Mr protein, but not sufficiently to explain their uncoupling action. The sensitivity of the fatty acid modulation of permeability is dependent on chain length, extent of unsaturation and pH. There is a requirement for an unesterified carboxyl group. In respiring mitochondria fatty acids act in the presence of nucleotide by lowering the 'break-point' potential at which the conductance of the 32000-Mr protein increases. Fatty acids have no effect on the chloride uniport activity of the 32000-Mr protein, but decouple the interference between chloride and protons when the simultaneous transport of both ions is attempted.

Photosynthetic pathways in the Bromeliaceae of Trinidad: relations between life-forms, habitat preference and the occurrence of CAM.

Abstract: An investigation was carried out into the photosynthetic pathways of the complete bromeliad flora of Trinidad (West Indies). Carbon-isotope ratios (δ13C values) were used to distinguish obligate C3 and crassulacean acid metabolism (CAM) species. Measurements were also carried out on some species in the field to test for day-night changes in leaf titratable acidity. A wide range of δ13C values was found. The obligate CAM species had values of -10 to -20‰ and the obligate C3 species of -23 to -35‰ CAM was found (a) in the majority of Tillandsia spp. (Tillandsioideae) and (b) in all species of Bromelioideae. The other genera of the Tillandsioideae appeared to be at least predominantly C3. One species, Guzmania monostachia var. monostachia, was identified as a C3-CAM intermediate, and others may well exist in the Trinidad flora. The influence of factors such as source CO2, photosynthetic photon flux density and ambient humidity in determining the δ13C values is discussed. The taxonomic distribution of C3 and CAM species within the Bromeliaceae is analyzed in terms of the life-forms and ecological types recognized by Pittendrigh (1948). The most xerophytic species (the light-demanding “atmospherics”) all show CAM and are restricted to the drier parts of the island. Most of the species with waterstoring “tanks” have a wide geographic distribution: these include light-demanding C3 plants and less light-demanding CAM plants. The shade-tolerant bromeliads, which show a requirement for high ambient humidity, are all C3 plants. We discuss the phylogenetic origins of CAM and the epiphytic habit in the Bromeliaceae.

The calmodulin-dependent glycogen synthase kinase from rabbit skeletal muscle Purification, subunit structure and substrate specificity

Abstract: A calmodulin-dependent glycogen synthase kinase distinct from phosphorylase kinase has been purified approximately equal to 5000-fold from rabbit skeletal muscle by a procedure involving fractionation with ammonium sulphate (0-33%), and chromatographies on phosphocellulose, calmodulin-Sepharose and DEAE-Sepharose. 0.75 mg of protein was obtained from 5000 g of muscle within 4 days, corresponding to a yield of approximately equal to 3%. The Km for glycogen synthase was 3.0 microM and the V 1.6-2.0 mumol min-1 mg-1. The purified enzyme showed a major protein staining band (Mr 58 000) and a minor component (Mr 54 000) when examined by dodecyl sulphate polyacrylamide gel electrophoresis. The molecular weight of the native enzyme was determined to be 696 000 by sedimentation equilibrium centrifugation, indicating a dodecameric structure. Electron microscopy suggested that the 12 subunits were arranged as two hexameric rings stacked one upon the other. Following incubation with Mg-ATP and Ca2+-calmodulin, the purified protein kinase underwent an 'autophosphorylation reaction'. The reaction reached a plateau when approximately equal to 5 mol of phosphate had been incorporated per 58 000-Mr subunit. Both the 58 000-Mr and 54 000-Mr species were phosphorylated to a similar extent. Autophosphorylation did not affect the catalytic activity. The calmodulin-dependent protein kinase initially phosphorylated glycogen synthase at site-2, followed by a slower phosphorylation of site-1 b. The protein kinase also phosphorylated smooth muscle myosin light chains, histone H1, acetyl-CoA carboxylase and ATP-citrate lyase. These findings suggest that the calmodulin-dependent glycogen synthase kinase may be a enzyme of broad specificity in vivo. Glycogen synthase kinase-4 is an enzyme that resembles the calmodulin-dependent glycogen synthase kinase in phosphorylating glycogen synthase (at site-2), but not glycogen phosphorylase. Glycogen synthase kinase-4 was unable to phosphorylate any of the other proteins phosphorylated by the calmodulin-dependent glycogen synthase kinase, nor could it phosphorylate site 1 b of glycogen synthase. The results demonstrate that glycogen synthase kinase-4 is not a proteolytic fragment of the calmodulin-dependent glycogen synthase kinase, that has lost its ability to be regulated by Ca2+-calmodulin.

Characterisation of a reconstituted Mg-ATP-dependent protein phosphatase.

Abstract: Homogenous preparations of the catalytic subunit of protein phosphatase-1 and inhibitor-2 can be combined to produce an inactive enzyme that consists of a 1:1 complex between these two proteins. This species is indistinguishable from the Mg-ATP-dependent protein phosphatase in that preincubation with glycogen synthase kinase-3 and Mg-ATP is required to generate activity. Activation results from the phosphorylation of inhibitor-2. The molar concentrations of protein phosphatase-1 and inhibitor-2 in rabbit skeletal muscle (0.25–0.5 μM) are similar. Incubation of the reconstituted Mg-ATP-dependent protein phosphatase with chymotrypsin is accompanied by limited proteolysis of inhibitor-2 and the loss of its phosphorylation site(s). This species can be activated by glycogen synthase kinase-3 and Mg-ATP provided that inhibitor-2 is added. This exogenous inhibitor-2 appears to displace the fragments of inhibitor-2 from the enzyme that were generated by chymotryptic digestion. These experiments may explain the report [Yang, S. D., Vandenheede, J. R. and Merlevede, W. (1981) J. Biol. Chem. 256, 10231–10234] that inhibitor-2 can function as an ‘activator’ as well as an inhibitor of the Mg-ATP-dependent protein phosphatase. Incubation of the catalytic subunit of protein phosphatase-1 with sodium fluoride or sodium pyrophosphate converted the enzyme to an inactive from that could be partially reactivated by manganese ions, but not by glycogen synthase kinase-3 and Mg-ATP. Conversely, the reconstituted Mg-ATP-dependent protein phosphatase could only be activated by glycogen synthase kinase-3 and Mg-ATP, and not by manganese ions. It is concluded that the conversion of protein phosphatase-1 to a manganese-ion dependent form is a quite separate phenomenon from the formation of the Mg-ATP-dependent protein phosphatase. Inhibitor-2 can inactivate protein phosphatase-1 by a second mechanism that is not reversed by preincubation with glycogen synthase kinase-3 and Mg-ATP. This occurs at higher concentrations of inhibitor-2 than those required to form the Mg-ATP-dependent protein phosphatase, and appears to result from the binding of inhibitor-2 to a distinct site on the enzyme.

The protein phosphatases involved in cellular regulation. 2. Glycogen metabolism.

Abstract: The nature of protein phosphatases that are active against the phosphorylated proteins of glycogen metabolism was investigated in rabbit skeletal muscle and liver. Six 32P-labelled substrates corresponding to the major phosphorylation sites on glycogen phosphorylase, phosphorylase kinase, glycogen synthase and inhibitor-1 were used in these studies. The results showed that the four protein phosphatases defined in the preceding paper, namely protein phosphatases-1, 2A, 2B and 2C [Ingebritsen, T. S. and Cohen, P. (1983) Eur. J. Biochem. 132, 255-261] were the only significant enzymes acting on these substrates. The four enzymes can be conveniently separated and identified by a combination of ion-exchange chromatography and gel filtration and by the use of specific inhibitors. Three species of protein phosphatase-2A were resolved on DEAE-cellulose, termed protein phosphatases-2Ao (0.12 M NaCl), 2A1 (0.2 M NaCl) and 2A2 (0.28 M NaCl) that had apparent molecular weights of 210000, 210000 and 150000 respectively. Protein phosphatase-2Ao was a completely inactive enzyme whose activity was only expressed after dissociation to a 34000-Mr(app) catalytic subunit by freezing and thawing in 0.2 M 2-mercaptoethanol. This treatment also dissociated protein phosphatases 2A1 and 2A2 to more active 34000-Mr(app) catalytic subunits. The catalytic subunits derived from protein phosphatases-2Ao, 2A1 and 2A2 possessed identical substrate specificities, preferentially dephosphorylated the alpha-subunit of phosphorylase kinase, were unaffected by inhibitor-1 and inhibitor-2 and were inhibited by similar concentrations of ATP. The properties of protein phosphatases-2A1 and 2A2 were very similar to those of the catalytic subunits, except that they were less sensitive to inhibition by ATP. Protein phosphatase-2B was eluted from DEAE-cellulose in the same fraction as protein phosphatase-2Ao. These activities were resolved by gel filtration, the Mr(app) of protein phosphatase-2B being 98000. Protein phosphatase-2B was completely inhibited by 100 microM trifluoperazine, which did not affect the activity of protein phosphatase-2Ao or any other protein phosphatase. Freezing and thawing in 0.2 M 2-mercaptoethanol resulted in partial inactivation of protein phosphatase-2B. Protein phosphatase-2C was eluted from DEAE-cellulose at the leading edge of the peak of protein phosphatase-2A1. These activities were completely resolved by gel filtration, since the Mr(app) of protein phosphatase-2C was 46000. Two forms of protein phosphatase-1 can be identified by chromatography on DEAE-cellulose, namely protein phosphatase-1 itself and the Mg X ATP-dependent protein phosphatase. Both these species were eluted at 0.16 M NaCl just ahead of protein phosphatases-2C and 2A1. These enzymes did not interfere with measurements of type-2 protein phosphatases, since it was possible to block their activity with inhibitor-2...

Physiological and bioenergetic aspects of mitochondrial calcium transport

Abstract: It is, we hope, apparent from this review that the past five or six years have been a considerable advance in our understanding of the mechanisms and regulation of calcium transport by the isolated mitochondrion. As a result, a field which has tended, for good reason, to be regarded by many physiologists as an artifact of isolated mitochondriology, should now be reexamined as a relevant factor in the complexities of cellular calcium regulation. In particular it would appear to be of prime importance to apply recently developed techniques to resolve the many anomalies concerning the role of mitochondria in specific cellular events.

The study of transport and related properties of amorphous silicon by transient experiments

Abstract: Transient experiments, particulary drift mobility measurements, have made valuable contributions to the understanding of electronic transport in amorphous (a-) semiconductors. The paper begins with a review of recent work on carrier mobilities in a-Si and a discussion of dispersive behaviour in relation to the tail state distributions. We then turn to the present controversy over the interpretation of transient signals and suggest that the conclusions of Silver et al cannot be upheld. The final part deals with information from transient experiments on carrier lifetimes and μτ-products in a-Si and a possible explanation for their thickness dependence is given. Further evidence is presented that neutral dangling bonds (D0) are the predominant sites for deep trapping.

Uptake and loss of nitrite from the blood of rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L. in fresh water and in dilute sea water

Abstract: The acute toxicity of nitrite (NO−2) to salmonids is strongly ameliorated by chloride (Cl−) ions rendering it almost harmless in most fresh waters apart from those with low Cl− content. In Cl− poor fresh water external NO−2 is concentrated in the blood plasma until it is at approximately the same molar concentration as haemoglobin (about 8 mmol) and at this point most of the haemoglobin has been oxidized to methaemoglobin this being a contributory cause of death. Two theories are advanced to account for NO−2 concentration in the blood. The first supposes that gills are impermeable to NO−2 but allow its conjugate acid nitrous acid (HNO2) to diffuse into the blood where it dissociates according to the blood pH value. Thus NO−2 will accumulate in the blood plasma if it has a higher pH value than the water. The second supposes that the Cl− uptake mechanism in the freshwater gill has an affinity for NO−2 and accounts for the fact that NO−2 entry to the blood is suppressed when external Cl− is present in significant amounts. The results also suggest that NO−2 and Cl− behave similarly as diffusing ions. Thus NO−2 diffusion into the blood of seawater fish and from the blood of NO−2 loaded freshwater fish occurs at approximately the same rate as the corresponding Cl− fluxes. Nitrite loss from seawater fish is thought to be mainly by diffusion although there is some evidence for the active Cl− extrusion mechanism having a weak affinity for nitrite.

Memory switching in amorphous silicon devices

Abstract: Recent experimental observations on high-speed memory switching in p+-n-i structures of amorphous silicon are described. Particular emphasis is given to the first switching operation of a virgin device which is effectively a unique forming process for all subsequent operations. There is a characteristic delay for forming, varying over ten orders of magnitude from ∼ 102to ∼ 10−8 s. Evidence is presented to show that forming is a charge controlled process which occurs at a constant field across the n-layer, but details of the switching mechanisms in the a-Si p+-n-i devices remain obscure.

The characteristics and properties of optimised amorphous silicon field effect transistors

Abstract: A series of experiments aimed at improving the performance of amorphous silicon field effect transistors has been carried out. The dc and dynamic characteristics of the optimised devices are described. Stable devices capable of ON-currents of the order of 100 μA with OFF-currents ≃10−11 A can be fabricated which could, in principle, be used to address more than 1000 lines of a liquid crystal display. The properties of the highly conducting ON-state channel have also been studied. The field effect mobility, 0.3 cm2 V−1 s−1 at room temperature, has an activation energy of 0.1 eV at the higher gate voltages. The possible reasons for the improvement in performance over earlier devices are discussed.

Results of AO plate fixation of forearm shaft fractures in adults.

Abstract: A series of 111 forearm fractures in 108 individuals involving 177 individual bones, and treated by AO plating is reported. There were 18 different surgeons. Open fractures occurred in 24 per cent of limbs. Reviews at a mean of 3 years after the accident showed 97 per cent of bones to be solidly united, and satisfactory function to be achieved in 80 per cent of limbs. Deep infection occurred 6 times, and non-union in 7 bones. Cross-union developed in 6 patients, all of whom had sustained head injuries. Seven patients sustained operative nerve injuries. It is felt that the implant of choice at the moment is the small fragment Dynamic Compression Plate (DCP).

A kinetic analysis of the effects of inhibitor-1 and inhibitor-2 on the activity of protein phosphatase-1.

Abstract: The steady-state interaction between protein phosphatase-1 and its two inhibitor proteins was studied in vitro at low enzyme concentrations where the assumptions of the Michaelis-Menten equation appeared to be valid. Under these conditions, and in the absence of divalent cations, inhibitor-1 behaved as a mixed inhibitor using phosphorylase alpha as a substrate, whereas inhibitor-2 was a competitive inhibitor. The results demonstrate that inhibitor-1 and inhibitor-2 do not interact with protein phosphatase-1 in an identical manner. Inhibitor-1 was only a substrate for protein phosphatase-1 in the presence of Mn2+, and its dephosphorylation was inhibited competitively by inhibitor-2 (Kis = 8 nM). Inhibitor-1 did not inhibit its own dephosphorylation in the presence of Mn2+. Its Km as a substrate (190 nM) was very much higher than its Ki as an inhibitor (1.5-7.5 nM). The results are consistent with a model in which a single binding site for inhibitor-1 is present on protein phosphatase-1, distinct from the binding site for phosphorylase alpha. It is envisaged that the binding of inhibitor-1 to this site not only inhibits the dephosphorylation of other substrates but permits access of its phosphothreonine to the same catalytic group(s) responsible for the dephosphorylation of other substrates. G-substrate, a protein phosphorylated exclusively on threonine residues, did not inhibit the dephosphorylation of phosphorylase alpha and its dephosphorylation was potently inhibited by inhibitor-1 or inhibitor-2. The role of the phosphothreonine residue in inhibitor-1 is discussed in the light of these results.

Cyanobacteria-eukaryotic plant symbioses

Abstract: N2-fixing heterocystous cyanobacteria develop in symbiotic association with a small number of eukaryotic plant species belonging to the algae, fungi, liverworts, ferns, gymnosperms and angiosperm. When the free-living cyanobacteria develop in symbiosis, they become modified morphologically, physiologically and biochemically. The symbiosis are relatively specific, and among the changes which occur in the endophytic cyanobacteria are increases in the size of the vegetative cells, changes in the ultrastructure of the vegetative cells, a tendency for a reduction in the filamentous habit, an increased heterocyst frequency when another photosynthetic partner is present, reduced activities of glutamine synthetase and glutamate synthase, and NH4+ release by; the endophytic cyanobacteria. These and other aspects are considered, emphasizing, in particular, work carried out in the authors' laboratory.

Fractured neck of femur. Pattern of incidence and implications.

Abstract: Projected figures for the 1975 incidence of fracture neck of femur in Dundee are compared with the actual figures. Analysis of the incidence in 1952,1975 and 1980, relating it to age and other factors, shows both an increase in the true incidence and a marked rise in the total number over the period. the length of stay in an Orthopaedic ward is considerably increased if the patient then has to be admitted to a long-stay bed. This length of inpatient stay and the increased incidence continues to raise the demand on acute Orthopaedic female beds.

Competitive interactions amongst four species of Glomus on maize and onion

Abstract: Maize and onion plants were infected with four species of Glomus in all fifteen possible combinations. Growth of maize was enhanced similarly in all treatments whereas onion plants showed differential growth effects. Inocula containing more than one endophyte gave more consistent results, although spore yields were lower, in mixed inocula compared with inocula of single species. G. clarum spored prolifically throughout the treatments. Maize plants produced three times the number of spores compared with onions. G. clarum was a good competitor and G. caledonium much less efficient. Mixed inocula are suggested for field inoculations.

A comparison of objective and subjective measures of alcohol dependence as predictors of relapse following treatment.

Abstract: Shortly before discharge, 50 alcoholic inpatients completed an 'objective' measure of alcohol dependence, the Severity of Alcohol Dependence Questionnaire, and a 'subjective' measure, taken largely from questions previously used by Schaefer (1971). At six-month follow-up, there was a significant difference in the subjective measure between patients who had maintained harm-free drinking and those who had returned to some form of harmful drinking, but no significant relationship was observed for the SADQ. It is concluded that relapse following treatment is better predicted by the subjective measure than by the objective measure of dependence and the implications of this for theories of relapse are noted.

Protein phosphorylation and the control of glycogen metabolism in skeletal muscle

Abstract: Glycogen metabolism in mammalian skeletal muscle is controlled by a regulatory network in which six protein kinases, four protein phosphatases and several thermostable regulatory proteins determine the activation state of glycogen phosphorylase and glycogen synthase, the rate-limiting enzymes of this process. Thirteen phosphorylation sites are involved, twelve of which have been isolated and sequenced and shown to be phosphorylated in vivo. The effects of adrenalin and insulin on the state of phosphorylation of each site have been determined. The neural control of glycogen metabolism is mediated by calcium ions and involves phosphorylase kinase, and a specific calmodulin-dependent glycogen synthase kinase. The $\beta$ -adrenergic control of the system is mediated by cyclic AMP, and involves the phosphorylation of phosphorylase kinase, glycogen synthase and inhibitor 1 by cyclic-AMP-dependent protein kinase. Inhibitor 1 is a specific inhibitor of protein phosphatase 1, the major phosphatase involved in the control of glycogen metabolism. The stimulation of glycogen synthesis by insulin results from the dephosphorylation of glycogen synthase at sites (3a + 3b + 3c), which are introduced by the enzyme glycogen synthase kinase 3. The structure, regulation and substrate specificities of the protein phosphatases involved in glycogen metabolism are reviewed. Protein phosphatase 1 can exist in an inactive form termed the Mg-ATP-dependent protein phosphatase, which consists of a complex between the catalytic subunit and a thermostable protein termed inhibitor 2. Activation of this complex is catalysed by glycogen synthase kinase 3. It involves the phosphorylation of inhibitor 2 and its dissociation from the catalytic subunit. Protein phosphatase 2A can be resolved into three forms by ion exchange chromatography. These species contain the same catalytic subunit and other subunits that may have a regulatory function. Protein phosphatase 2B is a Ca $^{2+}$ -dependent enzyme composed of two subunits, A and B. Its activity is increased tenfold by calmodulin, which interacts with the A-subunit. The B-subunit is a Ca $^{2+}$ -binding protein that is homologous with calmodulin. Its N-terminus contains the unusual myristyl blocking group, only found previously in the catalytic subunit of cyclic-AMP-dependent protein kinase. Protein phosphatase 2C is a Mg $^{2+}$ -dependent enzyme that accounts for a very small proportion of the glycogen synthase phosphatase activity in skeletal muscle. It is likely to be involved in the regulation of other metabolic processes in vivo such as cholesterol synthesis. Recent evidence suggests that many of the proteins involved in the control of glycogen metabolism have much wider roles, and that they participate in the neural and hormonal regulation of a variety of intracellular processes.