Thermal kinetic differences between allelic isozymes of malate dehydrogenase (Mdh-B locus) of largemouth bass, Micropterus salmoides.

Two alleles are encoded at the malate dehydrogenase locus in largemouth bass, Micropterus salmoides. Populations in the extreme northern areas of the range of this fish are fixed or nearly fixed for the B1 allele, whereas populations in Florida are fixed for the alternative allele, B2. The MDH-B1B1 and MDH-B2B2 allelic isozymes were isolated by preparative starch gel electrophoresis and subjected to in vitro kinetic analyses. The apparent Km (oxaloacetate) for each of these allelic isozymes was determined at 25, 30, and 35 degrees C. The Km values for both isozymes increased with increasing temperature and were not significantly different from each other at 25 and 35 degrees C. However, at 30 degrees C the Km value for the MDH-B1B1 allelic isozyme was higher than that for the MDH-B2B2 isozyme (i.e., 5.4 X 10(-5) vs 3.3 X 10(-5)). These results are consistent with the hypothesis that the different environmental temperatures at different latitudes may be at least partially responsible for the north-south cline in Mdh-B allele frequencies.