Synthesis and content of a DNA-binding protein with lactic dehydrogenase activity are reduced by nerve growth factor in the neoplastic cell line PC12

We have previously demonstrated that synthesis of a 34 kD protein having specific, high affinity for single-stranded DNA (34kD-ssb protein), is markedly inhibited by nerve growth factor (NGF) in the neoplastic clonal cell line PC12. We report here that total content as well as mRNA for this protein are progressively reduced in PC12 cells undergoing mitotic arrest and morphological differentiation induced by NGF. It is also shown that binding of the 34K-ssb protein to ssDNA is fully inhibited by NADH but not by NAD+ or by several other nucleotides. Enzymatic tests on the possible NADH/NAD+-dependent dehydrogenase activity of the 34K-ssb protein have demonstrated that it has lactic dehydrogenase activity (LDH) with a specific activity comparable to that of rabbit muscle. Furthermore, the 34K-ssb protein has the same peptide mapping as LDH purified from rat muscle. Antibodies directed against the 34K-ssb protein cross-react with the rabbit muscle enzyme and, vice versa, antibodies raised against rabbit LDH cross-react with the 34K-ssb protein. It is concluded that the 34K-ssb protein is identifiable with the type M of LDH, although possible differences in primary structure of the two proteins may have escaped the present studies. We hypothesize that interaction of the PC12 lactic dehydrogenase with ssDNA occurs also in vivo, as indicated by the findings reported in the accompanying paper, and may be modulated by the cellular content of NADH which, in turn, is related to energy metabolism.