Gel isoelectric focusing of mouse lactate dehydrogenase: heterogeneity of the isoenzymes A4 and X4.

LDH of mouse organs (including testis) was investigated by isoelectric focusing in polyacrylamide gels. The number of LDH bands in this pattern considerably exceeds the five (six in testis) of the standard electrophoretic pattern. An attempt was made to identify these bands as tetrameric isoenzymes formed by random association of different subunits. This included isoelectric focusing of purified LDH A, B, and X, two-dimensional separation of LDH, urea treatment of LDH, staining with specific substrates, and comparison of different organs. Further experiments were performed to exclude artifacts possibly produced by the isoelectric focusing technique. Different strains of mice were also investigated. The results demonstrate that in addition to the common five LDH bands (A4-B4) one set of five bands is formed by LDH A (A41-A42) and another one by LDH X (X 41-X42). Moreover, an unusual band was found which has a lower molecular weight and no affinity to the other isoenzymes. The data suggest that the heterogeneity of the LDH pattern revealed by isoelectric focusing arises from post-transcriptional events rather than from a number of additional genes.