Interaction between rat lactic dehydrogenase M4 isozyme and vitamin B2 derivatives

In an attempt to clarify the mechanism of LDH protein reaction with FMN, amino acid residues in the protein moiety such as histidine, arginine, tyrosine and tryptophan were chemically modified so that their effects on LDH activity and FMN reaction could be investigated. Although it was recognized that histidine and arginine residues participate in the catalytic action, and that these residues exist at the binding site of coenzyme and substrate, no reaction of FMN with these residues was noted. LDH was slightly inactivated when tryptophan residue was modified, but inhibition of this activity was completely prevented when FMN was allowed to react with zymoprotein in advance. As is true with native LDH, LDH with a modified histidine residue or tyrosine residue reacted with FMN, but LDH with a modified tryptophan residue did not so react. It was therefore shown that the tryptophan residue of native LDHM4 zymoprotein reacts with FMN.