The allelic isozymes of hexose-6-phosphate dehydrogenase isolated from Fundulus heteroclitus: physical characteristics and kinetic properties

Hexose-6-phosphate dehydrogenase (H6PDH-A2; beta-D-glucose:NAD(P)+oxido-reductase; E.C. 1.1.1.47) of the teleost Fundulus heteroclitus (L.)shows clinal allelic variation along the east coast of North America. Threeof the major allelic isozymes have been purified and compared for nativemolecular weight, subunit molecular weight, isoelectric point, thermalstability, and steady-state kinetic properties (pH 8.0 and 25 degrees C).Significant differences were found among the allelic isozymes forisoelectric point, thermal stability, and some kinetic parameters. Thepredominant allelic isozyme in northern populations (H6PDH-AcAc) was foundto be more sensitive to heat denaturation than were the predominanthomozygous allelic isozymes isolated from southern populations (H6PDH-AaAaand H6PDH-AbAb). The H6PDH-AcAc allelic isozyme had both a significantlygreater Km for glucose-6-phosphate than did either of the southernphenotypes and a significantly greater Km for NADP+ and Ki of NAD+ than didone of the southern phenotypes (H6PDH-AaAa). While the allelic isozymes arefunctionally nonequivalent, it is not yet known whether these differencesare reflected at higher levels of biological organization.