Replacement of unlabeled γ-butyrobetaine with γ-[2,3,4-2H6]butyrobetaine has a profound effect on the stoichiometry between decarboxylation of 2-oxoglutarate and hydroxylation in the reaction catalyzed by human γ -butyrobetaine hydroxylase. The ratios between decarboxylation and hydroxylation are 1.16 with Unlabeled and 7.48 with deuterated γ-butyrobetaine as substrate. From these ratios an internal isotope effect of 41 has been calculated. DV in the overall reaction measured as 2- oxoglutarate decarboxylation is 2.5 and DV/K is 1.0. For γ-butyrobetaine hydroxylase from Pseudomonas sp. AK 1, 2-oxoglutarate decarboxylation exceeds hydroxylation with 10% when deuterated γ-butyrobetaine is used. No excess was found with unlabeled substrate and no internal isotope effect could be calculated. DV for the bacterial enzyme is 6.

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