Failure to detect beta-leucine in human blood or leucine 2,3-aminomutase in rat liver using capillary gas chromatography-mass spectrometry

It has been reported (Poston, J. (1976) J. Biol. Chem. 251, 1859-1863; (1982) 255, 10067-10072; (1984) 259, 2059-2061) that mammalian tissues contain an adenosylcobalamin-dependent enzyme, leucine 2,3-aminomutase, which catalyzes the interconversion of beta-leucine and leucine. It was also reported that beta-leucine is detectable in normal human serum (mean = 4.8 mumol/liter, n = 37) and is elevated in serum from patients with cobalamin deficiency (mean = 24.7 mumol/liter, n = 17). Serum levels of leucine were claimed to be decreased in the cobalamin deficient patients (mean = 52 mumol/liter) as compared with the normal subjects (mean = 81 mumol/liter). It was also reported that rat liver supernatant catalyzed the formation of beta-leucine, leucine, or both amino acids from iso-fatty acids, and that the generation of leucine from iso-fatty acids was stimulated by adenosylcobalamin and inhibited by unsaturated cobalamin-binding protein. We have synthesized t-butyldimethylsilyl derivatives of beta-leucine and leucine and have used capillary gas chromatography-mass spectrometry for their analysis. Using forms of beta-leucine and leucine that contain several deuterium atoms in place of several hydrogen atoms as internal standards, techniques have been developed which make it possible to detect and quantitate as little as 0.1 mumol/liter of beta-leucine or leucine in human serum and in incubations containing rat liver supernatant. beta-Leucine was not detectable, i.e. less than 0.1 mumol/liter, in any sera from 50 normal human subjects or in any sera from 50 cobalamin-deficient patients. The mean level of leucine in the 50 cobalamin-deficient sera was 219 mumol/liter, which was not decreased with respect to that in the 50 control sera (167 mumol/liter). Experiments in which beta-leucine, leucine, isostearic acid, or isocaproic acid were incubated with rat liver supernatant in the presence or absence of adenosylcobalamin or cobalamin-binding protein failed to demonstrate the formation of leucine or beta-leucine or their interconversion under any of the conditions studied. We conclude that beta-leucine is not present in human blood and that the existence of leucine 2,3-aminomutase in mammalian tissues remains to be established.