Regulation of enzymes of carbohydrate metabolism during anoxia in the salt marsh bivalve Geukensia demissus
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 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 pyruvate dehydrogenase (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 of anoxia 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 gill the ratio (determined at 0.75 & 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 % a in controls to 65-75% a 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 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 also did the %a in adductor; for example, in adductor, active GPa content rose from 0.09 + 0.02 U/g wet weight in controls to 0.24 + 0.01 U/gww after 2 h and 0.16 + 0.02 U/gww after 12 h anoxia exposure. GS behaved oppositely in hepatopancreas showing a significant decrease in total activity in 12 h anoxic tissue but GS was unaffected by anoxia in gill or adductor muscle. In mantle, oppositely directed changes in total GS activity and the % active resulted in no change in the activity of the active I form during anoxia.