The beta 1 beta 1 and beta 2 beta 2 human liver alcohol dehydrogenase isoenzymes differ by only one residue at the coenzyme-binding site; Arg-47 in beta 1 is replaced by His in the beta 2 subunit. Since Arg-47 is thought to facilitate the carboxymethylation of Cys-46 in horse liver alcohol dehydrogenase by binding halo acids in a Michaelis-Menten complex prior to inactivation, the specificity and kinetics of modification of the two human liver beta beta isoenzymes with iodoacetate were compared. Both of the beta beta isoenzymes were inactivated by treatment with iodo[14C]acetate, and one Cys per subunit was carboxymethylated. Cys-174, which is a ligand to the active-site zinc atom in horse liver alcohol dehydrogenase, was selectively carboxymethylated in each of the human beta beta isoenzymes; less than 15% of the iodo[14C]acetate incorporated into the enzyme appeared in Cys-46. Therefore, the three-dimensional structure of the basic amino acids in the anion-binding site of the human beta beta isoenzymes appears to be different from that of horse liver alcohol dehydrogenase. The kinetics of alkylation are consistent with the formation of a Michaelis-Menten complex before inactivation of the isoenzymes. The average Ki values for iodoacetate were 10 and 16 mM for beta 1 beta 1 and beta 2 beta 2, respectively, and maximal rate constants for inactivation were 0.22 and 0.17 min-1, respectively. From these data, it can be concluded that there is a relatively minor effect of the substitution of His for Arg at position 47 on the kinetics of inactivation.