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Overton and coworkers discovered a leucine 2,3-aminomutase in plant tissue cultures of Andrographis paniculata that converts (S)-leucine in (R)-i-leucine [43] (Scheme 1.6.10). The enzyme activity was investigated in cell free extracts by incubation with (S)-[U-14C]leucine and by measuring the radioactivity of the methyl ester camphanamide derivatives of the reaction mixtures by radio-GC. The stereochemistry of the ¡-amino acid was determined by radio-GC comparison of the enzyme reaction product as methyl ester camphanamide derivative with an authentic sample. The enzyme is not dependent on cobalamin, because addition of intrinsic factor does not induce its inhibition.

Scheme 1.6.10. Biosynthesis of ^-¡-Leucine 37 in Andrographis paniculata.

The presence of B12-dependent leucine 2,3-aminomutases in bacteria, mammals, and plants has also been reported [44-46]. Their existence was deduced only indirectly. B12 itself has not yet been detected in plants, however. Later Stabler [47] and Aberhart [48] reported independently they were unable to detect ¡-leucine and leucine 2,3-aminomutase activity in human blood and rat liver. The presence in these of B12-dependent leucine 2,3-aminomutases is therefore questionable.

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