A multilocus system for studying tissue and subcellular specialization. The pH and temperature dependence of the two major NADP-dependent isocitrate dehydrogenase isozymes of the fish Fundulus heteroclitus.

Abstract

In the teleost fish Fundulus heteroclitus, there are three NADP-dependent isocitrate dehydrogenase isozymes. IDH-B2 is the only cytoplasmic isozyme, and IDH-C2 dominates the mitochondria of all tissues other than liver, where IDH-A2 is expressed. Since fish are ectotherms, their intracellular temperature and pH change directly with environmental temperature. In order to evaluate the influence of these environmental parameters on a model fish NADP-isocitrate dehydrogenase system, the major cytoplasmic (IDH-B2) and mitochondrial (IDH-C2) isozymes were kinetically evaluated as a function of pH and temperature. Whereas Vfmax and KmISOCm (where ISOC is isocitrate) were pH-independent, the Km for NADP was pH-dependent for both isozymes. The cytoplasmic isozyme (IDH-B2) had smaller KmNADP values between pH 7.0 and pH 8.0 than the mitochondrial form (IDH-C2). Vfmax and Km for substrate and coenzyme were temperature-dependent. Energy of activation for IDH-B2 and IDH-C2 was 10.6 and 12.8 kcal/mol, respectively. Both proteins had delta G not equal to values of about 15.8 kcal/mol, with significantly different distributions between delta H not equal to and delta S not equal to. The cytoplasmic isozyme (IDH-B2) appears to have a greater rate of catalysis than the mitochondrial enzyme (IDH-C2) at temperatures less than 30 degrees C. Moreover, the IDH-B2 isozyme had lower KmNADP values than the IDH-C2 isozyme at all temperatures, whereas the KmISOC values for the two isozymes were indistinguishable. Our data suggest that the two major NADP-dependent isocitrate dehydrogenase isozymes have unique physiological and metabolic functions that are adapted to the tissues and cellular compartments in which they are expressed.