Adipose-tissue pyruvate kinase. Properties and interconversion of two active forms

Abstract

1. Extraction of rat epididymal adipose tissue with buffer containing EDTA yields a pyruvate kinase, provisionally called PyK-A, the properties of which resemble in several respects those of the allosteric pyruvate kinase of liver. These properties include co-operative interactions with phosphoenolpyruvate, Mg2+, K+, NH4+ and ATP, and sensitivity to activation by fructose 1,6-diphosphate. 2. Extraction in the absence of EDTA yields predominantly a form, PyK-B, that shows both normal Michaelis–Menten kinetics with phosphoenolpyruvate, Mg2+ and ATP, and co-operative interactions with K+ and NH4+; this form is insensitive towards fructose 1,6-diphosphate. 3. Both forms yield simple kinetics with ADP, though Km values differ in the two systems. In all cases where co-operativity has been demonstrated, Hill-plot n values are between 1·4 and 2·0. 4. The conversion of PyK-A into PyK-B is mediated specifically by fructose 1,6-diphosphate; the reverse reaction is occasioned by EDTA, ATP or citrate. It is thought that a bivalent cation may be involved in this interconversion. 5. Attempts at partial purification have revealed that the enzyme resembles the pyruvate kinase of skeletal muscle, rather than that of liver, in its solubility in ammonium sulphate and elution from DEAE-cellulose. 6. The relevance of these properties in the regulation of pyruvate kinase activity in vivo in adipose tissue is discussed.