Biosynthesis of b-Alanine from Uracil

In mammals, uracil 2 and its precursor cytosine are transformed in a three step reaction to b-alanine 3 [1]. The enzymes involved are not very specific and also accept as substrates different 5-substituted pyrimidines, e.g. thymine which is similarly transformed to (R)-b-aminoisobutyric acid [(R)-10], as uracil is to b-alanine 3. This was shown by feeding experiments with 14C-labeled uracil and thymine, respectively [2, 3].

In the first step, the pyrimidine is hydrogenated at the double bond by dihy-drouracil dehydrogenase (EC to a dihydropyrimidine (Scheme 1.6.2). The enzymes obtained from rat [4] and human [5] liver have been purified and characterized. They were later subjected to molecular cloning [6]. The hydrogens are added to the double bond at the Si face of C5 and C6 in an anti-addition reaction. This was deduced from NMR spectra recorded from the isolated degradation products after administration of [5-2H]- and [6-2H]uracil and 2H2O to a mammalian enzyme system [7].

dihydropyrimidine dehydrogenase dihydropyrimidine dehydrogenase o


R = H: uracil (2) R = H: 5,6-dihydrouracil (5) R = CH3: thymine (7) R = CH3:(fl)-5,6-dihydrothymine (8)

I n2 amidohydrolase


R = H: 7V-carbamoyl-/0-alanine (6) R = H: /?-alanine (3) R = CH3: (7?)-./V-carbamoyl-/?-amino- R = CH3: [(R)-10]

isobutyric acid (9) Scheme 1.6.2. Biosynthesis of j-alanine 3 and (R)-jS-aminoiso-butyric acid [(R)-10], respectively by degradation of pyrimidines.

In the second step the dihydropyrimidines obtained are hydrolyzed by j-dihydropyrimidinase (EC to N-carbamoyl-j-alanine 6 and N-carbamoyl-j-aminoisobutyrate 9, respectively (Scheme 1.6.2). The enzyme isolated from rat liver was purified, characterized [8] and cloned [9].

In the third step, the carbamoyl group is cleaved, with loss of CO2 and NH3, to generate b-alanine 3 and (R)-b-aminoisobutyric acid [(R)-10], respectively, by N-carbamoyl-b-alanine amidohydrolase (EC (Scheme 1.6.2) [10]. The enzyme purified from rat liver is a hexamer [11].

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