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PubMed2001

The content of glycogen phosphorylase and glycogen in preparations of sarcoplasmic reticulum-glycogenolytic complex is enhanced in diabetic rat skeletal muscle.

AIMS/HYPOTHESIS: We have examined the effect of diabetes and pharmacological insulin treatment on the content of glycogen phosphorylase and glycogen associated with the sarcoplasmic reticulum-glycogenolytic complex from rat skeletal muscle. METHODS: Diabetes was induced in rats by streptozotocin injection. Enzymatic activities were measured using spectrophotometric methods. Glycogen phosphorylase was determined measuring the pyridoxal-5' -phosphate content and using polyacrylamide gel electrophoresis. Glycogen content was measured by enzymatic and the phenol sulfuric methods. RESULTS: The content of glycogen phosphorylase associated with the sarcoplasmic reticulum glycogenolytic complex gradually arises after diabetes induction. The content of glycogen phosphorylase was restored to a control value by pharmacological insulin treatment. In addition, the content of glycogen in preparations of sarcoplasmic reticulum-glycogenolytic complex of diabetic animals was also increased, whereas the content of glycogen in total muscle of diabetic rats was similar to that of the control rats. The absolute and relative amount of glycogen associated with sarcoplasmic reticulum seemed to increase in diabetic animals. These effects on the compartmentalisation of glycogen were suppressed by insulin treatment. Additionally, the rate of conversion of glycogen phosphorylase b to a, an index of the phosphorylase kinase activity, was 50 % lower in diabetic rats, increasing the dephosphorylated form of glycogen phosphorylase and, as a consequence, its association with sarcoplasmic reticulum membranes. CONCLUSION/INTERPRETATION: These results suggest that under diabetic conditions, both glycogen phosphorylase and a small percentage of muscle glycogen are relocalized in the sarcoplasmic reticulum-glycogenolytic complex.

Animals↗

PubMedDate not supplied

Enzymatic activity of glycogen metabolism in chorionic villi.

Glycogen content and six major enzymatic activities involved in glycogen metabolism were analysed in chorionic villi (CV). Glycogen levels were found to be lower than those known to exist in liver and muscle. Activities of alpha-glucosidase, amylo-1,6-glucosidase, phosphorylase b and phosphorylase kinase were detectable by standard methods. The enzymatic activities of glucose-6-phosphatase and phosphorylase a were undetectable. These findings suggest that CV biopsies can be useful for first-trimester diagnosis of glycogen storage disease types II, III and VI, but not for type I (glucose-6-phosphatase deficiency).

Chorionic Villi↗

PubMed1996

Variable conformation and dynamics of calmodulin complexed with peptides derived from the autoinhibitory domains of target proteins.

Calcium-saturated calmodulin (CaM) can bind and activate many target proteins through the direct association with the respective autoinhibitory domains. The CaM binding sequences within the autoinhibitory domains of these proteins have little sequence homology, and the mechanisms associated with CaM's ability to recognize and productively bind with these variable sequences is unclear. Common structural features of CaM bound to five peptides that are homologous to the autoinhibitory domains of smooth muscle myosin light chain kinase, CaM-dependent protein kinase II alpha, the plasma membrane Ca-ATPase, a MARCKS homolog, and glycogen phosphorylase kinase were assessed using frequency-domain fluorescence spectroscopy. In addition, the structural features of CaM complexed with the peptide melittin was also considered. We observe similar decreases in the average fluorescence lifetime and similar increases in the solvent accessibility of N-(1-pyrenyl)maleimide (PM) bound at Cys27 in calcium binding loop I in the amino terminal domain of CaM upon association with all six target peptides. Likewise, using fluorescence resonance energy transfer to measure the spatial separation between the opposing globular domains in CaM, we observe a similar spatial separation between the opposing globular domains of CaM bound to all six peptides. This indicates that CaM undergoes comparable structural changes upon association with all six target peptides. However, there are significant differences in the observed lifetime, solvent accessibility, correlation time associated with the segmented rotational motion of PM-CaM, and in the spatial separation between the opposing globular domains in CaM upon association with the individual target peptides, which indicates that CaM adopts a different tertiary structure that is dependent on the structural features of the bound target peptide. The correlation times associated with the overall hydrodynamic properties of CaM complexed with all six peptides are nearly identical (phi 2 approximately 10.6 +/- 0.4 ns) and are consistent with the known dimensions of CaM complexed to a peptide homologous to the CaM binding sequence of CaM-dependent protein kinase II alpha. Therefore, while these results are consistent with a common binding mechanism between CaM and all six target peptides, they indicate that the binding domains of CaM adopt different tertiary structures that allow them to bind with the variable sequences found in the autoinhibitory domains of target proteins with high affinity.

Amino Acid Sequence↗

PubMed1987

The regulatory and basal phosphorylation sites of hormone-sensitive lipase are dephosphorylated by protein phosphatase-1, 2A and 2C but not by protein phosphatase-2B.

The activity of hormone-sensitive lipase, the rate-limiting enzyme in adipose tissue lipolysis, is controlled by cAMP-mediated phosphorylation at a specific regulatory phosphorylation site. The lipase is also phosphorylated at a site, termed basal, without any effects on its activity [Strålfors et al. (1984) Proc. Natl Acad. Sci. USA 81, 3317-3321]. The capacity of protein phosphatase-1, 2A, 2B and 2C to dephosphorylate the lipase, selectively phosphorylated by glycogen synthase kinase-4 and cAMP-dependent protein kinase at the basal and regulatory phosphorylation sites, was compared with that towards glycogen phosphorylase and phosphorylase kinase (alpha subunit). Protein phosphatase-1, 2A and 2C were found to dephosphorylate both phosphorylation sites of hormone-sensitive lipase, while protein phosphatase-2B had no measureable activity towards any of the sites. When the activities of protein phosphatase-1, 2A and 2C were normalized with respect to the reference substrates, they were found to dephosphorylate the lipase regulatory site in the approximate relations of 1:4:3 and the basal site in the approximate relations of 1:6:4. Protein phosphatase-1 showed 20% higher and protein phosphatase-2A and 2C 80% higher activity towards the basal site compared to the regulatory site. The two phosphorylation sites of the lipase were comparable to good substrates for protein phosphatase-2A and 2C, but relatively poor substrates for protein phosphatase-1. Protein phosphatase-2C activity towards the lipase was completely dependent on Mg2+ with a half-maximal effect at 3 mM. Protamine increased the lipase dephosphorylation by protein phosphatase-1 3-5-fold with half-maximal effect at 0.6 microgram/ml, and by protein phosphatase-2A about 2-fold with half-maximal effect at 3-5 micrograms/ml, thus illustrating the potential for control of these lipase phosphatase activities.

Adipose Tissue↗

PubMed1977

The effect of streptozotocin-induced diabetes and of insulin supplementation on glycogen metabolism in rat liver.

The effects of streptozotocin-induced diabetes and of insulin supplementation to diabetic rats on glycogen-metabolizing enzymes in liver were determined. The results were compared with those from control animals. The activities of glycogenolytic enzymes, i.e. phosphorylase (both a and b), phosphorylase kinase and protein kinase (in the presence or in the absence of cyclic AMP), were significantly decreased in the diabetic animals. The enzyme activities were restored to control values by insulin therapy. Glycogen synthase (I-form) activity, similarly decreased in the diabetic animals, was also restored to control values after the administration of insulin. The increase in glycogen synthase(I-form) activity after insulin treatment was associated with a concomitant increase in phosphoprotein phosphatase activity. The increase in phosphatase activity was due to (i) a change in the activity of the enzyme itself and (ii) a decrease in a heat stable protein inhibitor of the phosphatase activity.

Animals↗

PubMed1984

Enzymatic activities in thylakoid membranes, which form medium [32P]NDP and [32P]ATP from 32Pi. Polynucleotide phosphorylase and adenylate kinase.

Soluble chloroplast coupling factor 1 (CF1) and the ATP synthase complex, under uncoupled conditions, can form bound ATP from tightly bound ADP and medium Pi. This partial reaction is a powerful probe of the mechanism of ATP synthesis. During our study of the synthesis of bound ATP by CF1 other enzyme activities, which generate [32P]nucleotides from 32Pi, were characterized and controlled. Two enzymes present at significant levels in the preparations are polynucleotide phosphorylase and adenylate kinase. Polynucleotide phosphorylase (PNPase) was found both in thylakoid and CF1 preparations and catalyzed the formation of [beta-32P]ADP via its Pi----ADP exchange activity. The formation of [beta-32P]ADP during net photophosphorylation is attributable to adenylate kinase action on the [32P]ATP formed since hexokinase and glucose effectively block its production. In addition, PNPase also degraded RNA present in thylakoid preparations yielding all four [32P]nucleoside diphosphates. PNPase was also shown to catalyze a Pi----ATP exchange that is dependent on RNA primers and other cofactors.

ATP Synthetase Complexes↗

PubMed1976