Purification and characterization of the lactate dehydrogenase (LDH-B4) allozymes of Fundulus heteroclitus.

In order to evaluate whether structural differences exist between allelic variants of a B-type lactate dehydrogenase (LDH; L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) in the minnow Fundulus heteroclitus, the allozymes (LDH-Ba4, LDH-Ba/Bb, and LDH-Bb4) were purified to homogeneity by affinity chromatography. Each variant was characterized as to holoenzyme and subunit molecular mass, isoelectric point (pI), thermal and urea stability, and susceptibility to proteolysis. Differences in electrophoretic mobilities were due to a lower pI for LDH-Ba4 (pI = 6.6) than for LDH-Bb4 (pI = 7.2). Stability to inactivation by heat, urea, and proteolysis was in each case: LDH-Bb4 greater than LDH-Ba/Bb greater than LDH-Ba4. Inactivation by trypsin may involve the arginine-rich catalytic loop of lactate dehydrogenase (50). The results suggest that the allozymes differ in their conformational flexibility.