Mechanism and specificity of reconstitution of dimeric lactate dehydrogenase from Limulus polyphemus

D-Lactate dehydrogenase (EC 1.1.1.28) from Limulus polyphemus is a homodimer which is composed of identical subunits of Mr = 35 000. The enzyme may be reversibly denatured and dissociated at acid pH or in 6M guanidine X HCl. The sigmoidal time course of reactivation obeys a consecutive uni-bimolecular mechanism with k1 = 6 X 10(-4) S-1 and k2 = 1.3 X 10(-4) M-1 S-1 (20 degrees C) as first- and second-order rate constants. Cross-linking experiments with glutaraldehyde prove that reactivation and dimer formation run parallel. Joint "synchronous" reconstitution of the enzyme with dimeric porcine mitochondrial malate dehydrogenase (after denaturation in 6M guanidine X HCl) does not yield active hybrids. The unchanged kinetics of reactivation in the absence and presence of the prospective partner of hybridization prove that inactive hybrid intermediates may also be excluded. The absence of hybrids upon synchronous reconstitution of the two closely related dimeric NAD-dependent dehydrogenases clearly suggests that the assembly of nascent oligomeric proteins must be highly specific.