Abstract
Glucose-6-phosphate dehydrogenase in mullet liver (Mugil cephalus L.) occurs in 2 electrophoretically distinguishable forms, termed I and II. Both are tetramers (at neutral pH) with molecular weights greater than 200,000. At pH 9, each tetramer dissociates into two dimer molecules which retain catalytic activity. At intermediate pH values, an equilibrium tetramer ↔ dimer mixture probably occurs. Kinetic studies of Form I indicate that it is strongly inhibited by TPNH, which is competitive with respect to both G6P and TPN. Inhibition is greatly increased at high temperatures while enzyme-substrate and enzymeecoenzyme affinities greatly decrease. These properties strongly favor glucose-6-phosphate dehydrogenase function at low temperature, a situation that is partly compensated for during warm acclimation by an increase in the relative importance of Form II. Although Form II displays catalytic and regulatory properties which are qualitatively similar to those of Form I, it is able to outcompete Form I for limiting substrate at high temperatures.
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Communicated by T. R. Parsons, Vancouver
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Hochachka, P.W., Clayton-Hochachka, B. Glucose-6-phosphate dehydrogenase and thermal acclimation in the mullet fish. Marine Biology 18, 251–259 (1973). https://doi.org/10.1007/BF00347787
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DOI: https://doi.org/10.1007/BF00347787