REGULATION OF ENZYMES OF CARBOHYDRATE-METABOLISM DURING ANOXIA IN THESALT-MARSH BIVALVE GEUKENSIA-DEMISSUS

The effects of anoxia exposure (2 or 12 h at 5 degrees C) on the tissu e-specific responses by enzymes of carbohydrate metabolism were analyz ed in the mantle, gill, hepatopancreas, and adductor muscle of the int ertidal bivalve Geukensia demissus demissus. Reversible enzyme phospho rylation has been identified in other species of marine molluscs as a mechanism for coordinating the suppression of metabolic rate and the r edirection of carbon into fermentative pathways under anoxic condition s. The present study shows patterns of response response to anoxia by five enzymes of carbohydrate metabolism, including glycogen synthetase (GS) and pyruvate dehydrogenase (PDH), whose responses to anoxia have not before been analyzed in marine molluscs. Anoxia-induced changes i n properties, consistent with reversible phosphorylation modification of the enzymes, were found for pyruvate kinase (PK) and PDH in all tis sues and in selected tissues for glycogen phosphorylase (GP) and GS. H owever phosphofructokinase did not appear to be modified in any tissue during anoxia. Within 2 h of anoxia exposure, PK showed a sharp drop in the activity ratio (at subsaturating vs. saturating PEP concentrati ons) that indicated a stable modification of enzyme K-m for PEP; for e xample, in the gill, the ratio (determined at 0.75 and 7.5 mM PEP) fel l from 0.39 +/- 0.08 for aerobic controls to 0.08 +/- 0.01 after 2 h a noxia. The percentage of PDH in the active a form also dropped signifi cantly, in anoxia from 80%-84% PDHa in controls to 65%-75% PDHa in ano xic tissues. Changes in both of these enzymes are consistent with anox ia-induced metabolic rate suppression. By contrast, anoxia exposure in creased GP activity in the gill and adductor muscle, indicating a need for increased glycogenolysis during anoxia in these tissues. Total ph osphorylase (a + b) activity increased in both tissues, as did the per centage of a in the adductor; for example, in the adductor, active GPa content rose from 0.09 +/- 0.02 U/g wet wt in controls to 0.24 + 0.01 U/g wet wt after 2 h and to 0.16 +/- 0.02 U/g wet wt after 12 h of an oxia exposure. Glycogen synthetase behaved oppositely in hepatopancrea s, showing a significant decrease in total activity in 12-h anoxic tis sue, but GS was unaffected by anoxia in the gill Or the adductor muscl e. In the mantle, oppositely directed changes in total GS activity and the percentage active resulted in no net change in the activity of th e active I farm during anoxia.