gamma-butyrobetaine hydroxylase: primary and secondary tritium kinetic isotope effects

Primary and secondary tritium kinetic isotope effects have been determined for the reactions catalyzed by purified preparations of gamma-butyrobetaine hydroxylase obtained from Pseudomonas sp AK 1 and from calf liver. With [methyl-14C,(3R)-3-3H]-gamma-butyrobetaine as substrate, the bacterial hydroxylase was found to exhibit a primary T(V/K) of 1.3-1.5. This value was determined from measurements of either the specific activity of the medium 3H2O or of the ratio of 3H/14C in the residual gamma-butyrobetaine. Under identical conditions of analysis, the calf liver enzyme exhibited a primary T(V/K) of approximately 15. With [methyl-14C,(4R)-4-3H] gamma-butyrobetaine as substrate, a beta-secondary T(V/K) of 1.10 has been determined for the calf liver hydroxylase; this supports the existence in the reaction mechanism of an sp2-hybridized transition state. A large normal value of 1.31 for the alpha-secondary T(V/K), as derived from measurements with [methyl-14C,2,3-3H]-gamma-butyrobetaine, suggests that the motions of the primary and alpha-secondary hydrogens are coupled in the C-H cleavage step and resulting synchronous rehybridization. A chemical mechanism involving homolytic cleavage of the C-H bond at the position undergoing hydroxylation is proposed and discussed.