Phosphoglucose Isomerase in Teleostean Fish

The glycolytic enzyme phosphoglucose isomerase (D-glucose 6-phosphate ketol-isomerase, EC 5.3.1.4), which catalyses the interconversion of glucose 6-phosphate and fructose 6-phosphate, has been studied in a number of mammalian species, and it is now evident that the mammalian enzyme is coded for by a single autosomal gene locus. The mammalian enzyme is a dimer of molecular weight about 120000 and the specific activity of the purified enzyme is in the range 500-1000 units/mg at 30°C. Studies of amino acid compositions have revealed homologies between the enzymes from different species (Noltmann, 1972). The same major isoenzymes have been found to occur in the tissues of individual mammals, and this isoenzyme has been shown to be under the control of one gene locus. Here we describe some of the molecular properties of two genetically independent isoenzymes of phosphoglucose isomerase, recently described in some fish (Actinopterygians) (Avise & Kitto, 1973; Dando, 1973). The two electrophoretically distinct isoenzymes of phosphoglucose isomerase (termed isoenzymes PGI-A and PGI-B) have been shown to exhibit tissue-dependent distribution, e.g. isoenzyme PGI-B was found predominantly in conger (Conger conger L.) white muscle. whereas conger liver shows predominantly isoenzyme PGI-A, which is also found in heart, spleen and erythrocytes. In most Actinopterygian species isoenzyme PGI-B is less positively charged on electrophoresis at pH 6.8, and therefore moves more slowly towards the cathode than does isoenzyme PGI-A. In some species, and in certain tissues, hybrid PGI-A-PGI-B isoenzymes are observed if the two phosphoglucose isomerase gene loci are expressed in the same tissue (Dando, 1973), but this is not true for the conger. Molecular weights have been examined by using gel filtration through thin layers of Sephadex G-150 and we found that both liver and muscle enzymes had molecular weights in the range 130000-140000. Eight diverse Actinopterygian fish species all showed values in this range [Conger conger L., Gadus morhua L., Notopterus kapirat, Sprattus sprattus L., Scyliorhinus caniculus L., Branciostoma lanciolatum (Pallas), Lampetra sp. and Triturus sp.)}