Regulation of Enzymes of Carbohydrate Metabolism during Anoxia in the Salt Marsh BivalveGeukensia demissus

Abstract

The effects of anoxia exposure (2 or 12 h at 5°C) on the tissue-specific responses by enzymes of carbohydrate metabolism were analyzed in the mantle, gill, hepatopancreas, and adductor muscle of the intertidal bivalve Geukensia demissus demissus. Reversible enzyme phosphorylation has been identified in other species of marine molluscs as a mechanism for coordinating the suppression of metabolic rate and the redirection of carbon into fermentative pathways under anoxic conditions. The present study shows patterns of 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 in properties, consistent with reversible phosphorylation modification of the enzymes, were found for pyruvate kinase (PK) and PDH in all tissues and in selected tissues for glycogen phosphorylase (GP) and GS. However, phosphofructokinase did not appear to be modified in any tissue during anoxia. Within 2 h ofanoxia exposure, PK showed a sharp drop in the activity ratio (at subsaturating vs. saturating PEP concentrations) that indicated a stable modification of enzyme Km for PEP; for example, in the gill, the ratio (determined at 0.75 and 7.5 mM PEP) fell from 0.39 ± 0.08 for aerobic controls to 0.08 ± 0.01 after 2 h anoxia. The percentage of PDH in the active a form also dropped significantly in anoxia from 80%-84% PDHa in controls to 65%-75% PDHa in anoxic tissues. Changes in both of these enzymes are consistent with anoxia-induced metabolic rate suppression. By contrast, anoxia exposure increased GP activity in the gill and adductor muscle, indicating a need for increased glycogenolysis during anoxia in these tissues. Total phosphorylase (a + b) activity increased in both tissues, as did the percentage 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 anoxia exposure. Glycogen synthetase behaved oppositely in hepatopancreas, showing a significant decrease in total activity in 12-h anoxic tissue, but GS was unaffected by anoxia in the gill or the adductor muscle. In the mantle, oppositely directed changes in total GS activity and the percentage active resulted in no net change in the activity of the active I form during anoxia.