Vol. 30, Issue 12, 1436-1440, December 2002

Kinetics of Sequential Metabolism from D-Leucine to L-Leucine via -Ketoisocaproic Acid in Rat

Hiroshi Hasegawa, Takehisa Matsukawa, Yoshihiko Shinohara, and Takao Hashimoto

Department of Pathophysiology, School of Pharmacy, Tokyo University of Pharmacy and Life Science, Tokyo, Japan

D-Leucine is considered to be converted into the L-enantiomer by two steps: oxidative deamination to form -ketoisocaproic acid (KIC) and subsequent stereospecific reamination of KIC. We investigated the pharmacokinetics of leucine enantiomers and KIC in rats to evaluate how deamination of D-leucine, reamination of KIC, and decarboxylation of KIC were affected to the overall extent that converted D-leucine into the L-enantiomer. After intravenous administrations of D-[²H₇]leucine, L-[²H₇]leucine, or [²H₇]KIC, their plasma concentrations together with endogenous L-leucine and KIC were determined by gas chromatography-mass spectrometry. The rapid appearances of [²H₇]KIC and L-[²H₇]leucine were observed after administration of D-[²H₇]leucine, whereas no detectable amount of D-[²H₇]leucine was found after administrations of [²H₇]KIC or L-[²H₇]leucine. The fraction of conversion from D-[²H₇]leucine into [²H₇]KIC (F_DKIC) was estimated by using the area under the curve (AUC) of [²H₇]KIC on the D-[²H₇]leucine administration [AUC_KIC(D)] and that of [²H₇]KIC on the [²H₇]KIC administration (AUC_KIC) to yield 70.1%. The fraction of conversion from [²H₇]KIC to L-[²H₇]leucine (F_KICL) was 40.2%. The fraction of conversion from D-leucine to the L-enantiomer (F_DL) was considered to be the product of F_DKIC and F_KICL, indicating that 28.2% of D-[²H₇]leucine was metabolized to L-[²H₇]leucine via [²H₇]KIC. These results suggested that the relatively low conversion of D-leucine into the L-enantiomer might depend on irreversible decarboxylation of KIC. Regardless of [²H₇]KIC, F_DL was also calculated directly using AUC_L(D) and AUC_L to yield 27.5%. There were no differences between the two F_DL values, suggesting that almost all of the formation of L-[²H₇]leucine from D-[²H₇]leucine occurred via [²H₇]KIC as an intermediate.