Tissue specific isozymes of pyruvate kinase in the channelled whelkBusycotypus canaliculatum: enzyme modification in response to environmental anoxia

Abstract

1. Isoelectrofocusing and cellulose acetate electrophoresis demonstrated the presence of three tissue specific isozymic forms of pyruvate kinase (PK) occurring in red muscle (ventricle, radula retractor), white muscle (foot, mantle) and soft tissues (gill, kidney, hepatopancreas) of the channelled whelk,Busycotypus canaliculatum. 2. Under the assay conditions used the three isozymes differed kinetically with respect to fructose-1,6-bisphosphate activation: red muscle PK was insensitive, white muscle PK was slightly activated and soft tissue PK was strongly activated. 3. Kinetic and physical properties of the enzyme in all tissues were modified when PK was isolated from whelks exposed to 21 h of anoxia stress. Compared to PK from aerobic tissues, the enzymes from anoxic animals showed a decrease in pI of 0.4 pH unit and an increase in anodal migration on cellulose acetate. Anoxia reduced the maximal activity of PK in all tissues and increased the activity ratios at high versus low phosphoenolpyruvate (PEP) concentrations. White muscle and soft tissue PK from anoxic whelks also showed increased activation by fructose-1,6-biphosphate compared to aerobic animals. 4. Maximal activities and activity ratios at high versus low PEP for phosphoenolpyruvate carboxykinase (PEPCK) were not altered as a result of anoxia stress. 5. The anoxia-induced in vivo modification of PK appears to be the result of enzyme phosphorylation during anoxia and this covalent modification is probably a major factor limiting PK activity during anoxia and promoting the carboxylation of PEP via PEPCK.