Phosphorylase, activation

The cAMP formed by adenylyl cyclase (Figure 15.20) does not persist because 5 -phosphodiesterase activity prevalent in cells hydrolyzes cAMP to give 5 -AMP. Caffeine inhibits 5 -phosphodi-esterase activity. Describe the effects on glycogen phosphorylase activity that arise as a consequence of drinking lots of caffeinated coffee. 

Enzymes have evolved such that their values (or A o.s values) for substrate(s) are roughly equal to the in vivo concentration(s) of the substrate (s). Assume that glycogen phosphorylase is assayed at [P[] = A o.s in the absence and presence of AMP or ATP. Estimate from Figure 15.15 the relative glycogen phosphorylase activity when (a) neither AMP or ATP is present, (b) AMP is present, and (c) ATP is present. 

The gene for myophosphoiylase has been assigned to chromosome 1 lql3. The enzyme is a dimer of two identical 97 IcDa subunits and is the sole isoform present in skeletal muscle. Heart and brain also contain this isoform in addition to a distinct brain isoenzyme and a hybrid muscle/brain isoform. Smooth muscle also contains a phosphorylase isoform distinct from the muscle isoenzyme. If regenerating muscle fibers are present they also contain phosphorylase activity due to the presence, in fetal and developing muscle, of an isoform said to be identical with brain phosphorylase. 

Glycogen Synthase Phosphorylase Activity Are Reciprocally Regulated (Figure 18-7)... 

Not only is phosphorylase activated by a rise in concentration of cAMP (via phosphorylase kinase), but glycogen synthase is at the same time converted to the inactive form both effects are mediated via cAMP-dependent protein kinase. Thus, inhibition of glycogenolysis enhances net glycogenesis, and inhibition of glycogenesis enhances net glycogenolysis. Furthermore,... 

Figure 5. The increase in serum alkaline phosphatase activity (ALP) parallels the increase in trans-phosphorylase activity (TP) produced by increasing the molarity of 2-etnylaminoethanol ( EAE).

The chief loss in phosphorylase activity appeared to occur in the early stages of the fractionation the precipitated material remained active for several days in 0.6 saturated ammonium sulfate at 4°. The invertase was apparently inactivated by prolonged contact with 0.6 saturated ammonium sulfate, since it could not be recovered from the supernatant liquid after precipitation of the active material. 

Other stndies have examined the association between the activity of TPMT and other enzymes in the pnrine pathway and AZA toxicity. In one stndy, TPMT, HPRT, 5 -nncleotidase, and pnrine nncleoside phosphorylase activity in the RBCs of 33 RA patients on AZA (dose of approximately 2mg/kg/d) and 66 controls was meas-nred. Compared to patients with normal TPMT activity, 14 RA patients with low (p = 0.004) and 7 patients with intermediate TPMT activity (RR 3.1) developed AZA toxicity(4d). None of the patients were genotyped. Another study measured TPMT activity in 3 RA patients who had experienced AZA-induced hematologic toxicity and 16 RA patients withont AZA toxicity. In this study, 2 patients with AZA-indnced hematologic toxicity were TPMT deficient, one partial and the other complete (50). Patients were not genotyped in either of these studies. 

Figure 3.12 The regulation of phosphorylase activity by reversible phosphoiylation. A reversible phosphorylation process is also known as an interconversion cycle the latter term is preferred in this text, since the individual reactions must be irreversible, which can be confusing if the term reversible is used to describe the overall process. In resting muscle, almost all phosphorylase is in the b form.

Nucleoside phosphorylases that catalyse the reversible cleavage of purine nucleosides to the free bases and ribose-1-phosphate are found in most cells, although a phosphorylase that will cleave adenosine has so far been identified only in bacteria. Recent studies have shown that ribo- and 2 -deoxyribofurano-syltransferase activity is associated with phosphorylase activity [19, 23., 222] and that both activities reside in one enzyme, which can be converted from one form to the other by substrate or product binding [20]. Upon crystallization of the enzyme from human erythrocytes a marked decrease in the ribosyl transfer reaction was observed [21b]. 

Niridazole exhibits schistosomicide and amebicidal action. The mechanism of action is not known. It seems likely that it is concentrated in the parasite organism, which causes inhibition of phosphorylase activation, which is expressed in the depletion of glycogen reserves. 

Phosphorylase b-kinase converts the enzyme phosphorylase from its low activity form (phosphorylase b) to the high activity form (phosphorylase a) by phosphorylation. The enzyme activity of phosphorylase b-kinase is correlated with the increase of phosphorylase activity in the probands sample and is measured by determining the amount of P, produced by phosphorylase activity. 

Van Kuilenburg ABP, Zoetekouw L (2005) Determination of thymidine phosphorylase activity by a non-radiochemical assay using reversed-phase high-performance liquid chromatography. J Chromatogr Biomed Sci Appl 820 271-275... 

FIGURE 6-31 Regulation of glycogen phosphorylase activity by covalent modification. In the more active form of the enzyme, phosphorylase a, specific Ser residues, one on each subunit, are phosphorylated. Phosphorylase a is converted to the less active phosphorylase b by enzymatic loss of these phosphoryl groups, promoted by phosphorylase phosphatase. Phosphorylase b can be reconverted (reactivated) to phosphorylase a by the action of phosphorylase kinase. 

Alston, S., Hampson, L., Gomez-Foix, A.M., Gninovart, J.J., Aglus, L. (2001) Hepatic glycogen synthesis is highly sensitive to phosphorylase activity evidence from metabolic control analysis. J. Biol. Chem. 276, 23,858-23,866. 

Correct answer = B. Epinephrine and glucagon both cause increased glycogen degradation in the liver. Therefore, glycogen phosphorylase activity is increased, whereas glycogen synthase activity is decreased. Both cAMP-dependent protein kinase and its substrate, phosphorylase kinase, are also activated. 

A guanosine aminohydrolase preparation from Pseudomonas convexa No. 149 free from guanine, adenine and adenosine aminohydrolase and nucleoside phosphorylase activities has been described (63). Of the 22... 

Fig. 5.37. Modulation of glycogen phosphorylase activity by covalent modulation...

DiMarco JP, Miles W, Akhtar M, Milstein S, Sharma AD, Platia E, McGovern B, Scheinmman MM, Govier WC (1990) Adenosine for paroxysmal supraventricular tachycardia dose ranging and comparison with verapamil. Assessment in placebo-controlled, multicenter trials. The Adenosine for PSVT Study Group. Ann Intern Med 113(2) 104-110 Dobson JG (1978) Reduction by adenosine of the isoproterenol-induced increase in cyclic 3 , 5 -monophosphate formation and glycogen phosphorylase activity in rat heart muscle. Circ Res 43(5) 785-792... 

Fig. 2. Regulation of (a) glycogen phosphorylase activity and (b) glycogen synthase activity by phosphorylation (covalent modification).

Phosphorylase activity in spinach leaf is also distributed in the chloroplastic and cytoplasmic fractions, but in comparable amounts.46,47 The cytoplasmic enzyme has much higher affinity for... 

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