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Different ATP inhibition patterns are reported for both phosphorylated and unphosphorylated forms of glycogen phosphorylase from Mytilus galloprovincialis mantle. The inhibition of the 'b' form conforms to the hyperbolic mixed (i.e. partial uncompetitive) model with respect to substrates, glycogen and inorganic phosphate (P(i)). At low AMP concentrations, ATP and other nucleotides (ADP and IMP) have an activating effect on the unphosphorylated form. The inhibition of the 'a' form conforms to the hyperbolic mixed model with respect to glycogen, but to the simple competitive model with respect to P(i). The effects of ATP on the phosphorylated form appear to reverse the effect of enzyme phosphorylation. The influence of energy status on the activity of glycogen phosphorylase was also studied. Activity was maximal at high AMP and ATP concentrations at low and high energy charges (ECs), respectively. At low EC, the enzyme was activated by AMP, strongly inhibited by ADP and weakly inhibited by ATP. These results support the hypothesis that seasonal variations in EC control glycogen mobilization in Mytilus, related to gametogenic development and to basal metabolism in sexual resting periods. Copyright (C) 1998 Elsevier Science Inc. |
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