Inhibition kinetics of NAD-linked enzymes by gossypol acetic acid

Effects of gossypol on human spermatozoal enzymes were investigated. This compound was found to be a potent inhibitor of the NAD-linked enzymes, glucelaldehyde-3-phosphate dehydrogenase (GA3PDH), lactate dehydrogenase (LDH), malate dehydrogenase (MDH), and isocitrate dehydrogenase (ICDH). MDH was inhibited by gossypol when the reaction was carried out in malate-oxalacetate (direct) or oxalacetate-malate (reverse) directions. The I50 of gossypol for the direct reaction was 2.9 microM, whereas that for the reverse reaction was 1.2 microM. Reciprocal plots due to Lineweaver-Burk showed that MDH is inhibited in a noncompetitive manner with respect to both reactions. LDH was also inhibited by this compound when pyruvate or alpha-ketobutyrate was used as a substrate. Gossypol was a noncompetitive inhibitor for LDH. The I50 of gossypol for LDH were 9.8 microM and 11.3 microM, when using pyruvate and alpha-ketobutyrate, respectively as substrates. The I50 of gossypol for GA3PDH and ICDH were 110 microM and 2.7 microM, respectively.