3-HYDROXY-3-METHYLGLUTARYLDITHIO-COA - UTILITY OF AN ALTERNATIVE SUBSTRATE IN ELUCIDATION OF A ROLE FOR HMG-COA LYASES CATION ACTIVATOR

(S)-(3-Hydroxy-3-methyl-1-thionoglutaryl)-Coenzyme A (HMG[= S]CoA), a dithioester analog of (S)-(3-hydroxy-3-methyl-glutaryl)-CoA (HMG-CoA), acts as an efficient alternative substrate for avian HMG-CoA lyase. Detection of product formation by HPLC, UV absorbance and coupled enzyme assays indicates that HMG[= S]CoA cleavage yields acetyl[= S]CoA and acetoacetate. HMG[= S]CoA binds to the lyase with a K(m) of 13 muM and undergoes the cleavage reaction at a maximal rate which is 20% of that observed with HMG-CoA. The enzyme-catalyzed cleavage of both HMG-CoA and HMG[ = S]CoA is stimulated by the divalent cations Mg2+ and Mn 2+. Mg2+ produces a 2-fold higher stimulation of HMG-CoA cleavage than that observed with Mn2+. In contrast, stimulation of HMG[= S]CoA cleavage is nearly seven times higher with Mn2+ than with Mg2+. Not only is the stimulation of enzymatic activity dependent on the cation, but also the K(m) values for Mg2+ and Mn 2+ are dependent upon the substrate used. In contrast, the K(m) values for HMG-CoA and HMG[ = S]CoA are not markedly dependent on the identity of the divalent cation. These results are compatible with the initial formation of a binary enzyme-substrate complex prior to binding of the divalent cation to produce a catalytically active enzyme-substrate-metal ternary complex.