Regulation at the phosphoenolpyruvate branchpoint by the adenylate energy charge in Mytilus edulis.

Pyruvate kinase (EC 2.7.1.40) is an enzyme with high activity in most tissues. Phosphoenolpyruvate carboxykinase (EC 4.1 .I .32) always has a restricted distribution. In vertebrates phosphoenolpyruvate carboxykinase is found in tissues, such as liver and kidney cortex, with highgluconeogenicactivities (Scrutton & Utter, 1968). In thesetissues the enzyme operates together with pyruvate carboxylase in the ‘dicarboxylic acid shuttle’, which SeNes to generate phosphoenolpyruvate from non-carbohydrate sources under metabolic conditions in which the net flux of the Embden-Meyerhof pathway is towards glucose. Similarly, in many bacteria phosphoenolpyruvate carboxykinase is linked to biosynthetic pathways and serves, by the production of oxalacetate, to replace the tricarboxylic acid-cycle intermediates that are consumed in biosynthetic sequences (Liao & Atkinson, 1971a). In many invertebrates, however, phosphoenolpyruvate carboxykinase participates in degradative sequences and the enzyme is found with high activity in glycolytic tissues. This is especially true for the sea mussel Mytilus edulis in which the posterior adductor muscle shows the highest activity of all the body tissues (de Zwaan, 1972). Its role is to convert phosphoenolpyruvate into oxaloacetate, which in turn is converted into the glycolytic end products. Whatever the fate of phosphoenolpyruvate, conversion into glucose or into aerobic or anaerobic end products, the phosphoenolpyruvate branchpoint must be regulated when both pyruvate kinase and phosphoenolpyruvate carboxykinase are present in the same tissue or cell. It appears that in such tissues (liver, kidney cortex, posterior adductor muscle) as well as in bacterial species, pyruvate kinase shows allosteric behaviour and therefore is affected by many modulators (Llorenteetal., 1970; Liao &Atkinson, 1971a,b; deZwaan &Holwerda, 1972; Holwerda & de Zwaan, 1973). Atkinson (1968) postulated that at any point where a metabolite is partitioned between energy-yielding (degradat ive sequences) and energy-demanding (biosynthetic sequences) processes the energy charge ([ATP]++[ADP])/([ATP]+ [ADP]+ [AMP]) is a major regulatory factor. As far as the phosphoenolpyruvate branchpoint is concerned Azotobacter vinelandii is the only example known by us in which the two competing enzymes phosphoenolpyruvate carboxylase (EC 4.1.1.3 1) and pyruvate kinase from one species or tissue have been studied in relation to the energy charge. In this species, pyruvate kinase is activated by a decrease in the energy charge, whereas phosphoenolpyruvate carboxylase is not under (direct) control of this parameter. The last result is understandable, as none of the compounds that determine the energy-charge value participate in the reaction: