Lactate dehydrogenase of Xenopus laevis laevis and Xenopus borealis depends on a multiple gene system

Lactate dehydrogenase isozymes have been reinvestigated in Xenopus laevis laevis and Xenopus borealis. High resolution zymograms of various organs demonstrate that in both species the LDH isozymes are governed basically by a three-gene system: Ldh-a coding for positively charged polypeptide, and Ldh-b and Ldh-c for negatively charged polypeptides (at pH 8.9). These three LDH subunits when assembled in tetramers show differential sensitivity to heat inactivation; the C4 homotetramer is the most labile isozyme. Xenopus is thus similar to Osteichthyes, birds, and mammals that also have a three-gene system for LDH. With respect to tissue specific isozyme expression, Xenopus resembles the more primitive families of bony fish. Superimposed on this three-gene system is a probable gene duplication for both LDH-a and Ldh-b. Heterotetrameric isozymes are formed between the various subunits leading to multibanded zymograms with a total of at least 21 distinct zones of LDH activity. The expression of genes is tissue specific not only for the basic genes, but also for their presumed duplicates. Since both species of Xenopus are ancient tetraploids, duplication of Ldh genes is not surprising.