SELECTIVE-INHIBITION OF THE SPERM-SPECIFIC LACTATE-DEHYDROGENASE ISOZYME-C4 BY N-ISOPROPYL OXAMATE

In the present study, we demonstrated that the attachment of the nonpo lar isopropylic carbon chain in the nitrogen of oxamate, converted thi s competitive inhibitor of LDH isozymes into a powerful selective inhi bitor of mouse LDH-C4. The comparative study of the inhibitory effect of oxamate and N-isopropyl oxamate on mouse LDH isozymes pointed out t hat the isopropylic carbon chain conferred upon N-isopropyl oxamate a high affinity for LDH-C4 and a marked decrease in the affinity for the other isozymes since oxamate showed more inhibitory effect on LDH-1 ( K-i = 0.06 mM) and LDH-5 (K-i = 0.08 mM), and less inhibitory effect o n LDH-C4 (K-i = 0.25 mM). On the other hand, N-isopropyl oxamate showe d the highest inhibitory effect on LDH-C4 (K-i = 0.014 mM) and poor in hibitory effect on LDH-1 (K-i = 0.4 mM) and LDH-5 (K-i = 0.8 mM). Appa rently, the enzymatic inactivation proceeded through a reversible bind ing of N-isopropyl oxamate, facilitated by nonpolar interactions with a hydrophobic region present only in the active site of mouse LDH-C4, resulting in a selective inhibition of this isozyme in comparison with the other LDH isozymes. N-isopropyl oxamate was also a powerful compe titive inhibitor of LDH-C4 (K-i = 0.015 mM) compared with oxamate (K-i = 0.35 mM), using alpha-ketoisocaproate as a substrate. (C) 1997 Else vier Science B.V.