Pyrimidine metabolism and secondary product formation; biogenesis of albizziine, 4-hydroxyhomoarginine and 2,3-diaminopropanoic acid

Abstract

In relation to the biogenesis of pyrimidine-derived secondary products, the relative activities and substrate specificities of the enzymes of pyrimidine metabolism were determined in extracts of seedlings from a number of leguminous species. Carbamoyl phosphate synthetase (CPSase) and aspartate transcarbamoylase (ATCase) were 4 to 6 × more active, and 5′-nucleotidase and uridine hydrolase were 2 to 3 × more active in the secondary metabolite producing species examined. As it was also confirmed that UMP is the most effective nucleotide inhibitor of ATCase activity, these relatively high rates of hydrolysis of UMP to uracil would lessen effective negative feedback control of pyrimidine synthesis in the secondary metabolite producers and further increase the availability of uracil for secondary metabolism. The key enzyme of pyrimidine catabolism, NADPH-dependent dihydrouracil dehydrogenase was shown to use uracil, thymine, lathyrine and 5-aminouracil as substrates and its activity was 2 to 3 × higher in those species not yielding pyrimidine secondary products. This observation is consistent with the diversion, in the secondary product-forming plants examined, of significant amounts of uracil away from catabolism and into secondary product synthesis. The dihydrouracil dehydrogenase of Albizia julibrissin exhibited higher activity with 5-aminouracil as substrate than with uracil or thymine. Dihydropyrimidinase, the next enzyme in the catabolic pathway, was extracted from the various species and shown to use 5,6-dihydrouracil 5,6-dihydrothymine and 5-amino-5,6-dihydrouracil as substrates. Using a NADPH-regenerating system, the combined action of the two enzymes, dihydrouracil dehydrogenase and dihydropyrimidinase, was shown to produce 4-hydroxyhomoarginine from lathyrine. This was identified by co-chromatography and electrophoresis with authentic samples of 4-hydroxy-homoarginine. Preparations of β-ureidopropionase, the third enzyme in the catabolic pathway, were obtained from seedlings of Pisum sativum and shown to hydrolyse N-carbamoyl- β-alanine to 3-aminopropanoic acid, and N-carbamoyl-2-methyl-3-aminopropanoic acid to 3-amino-2-methylpropanoic acid. Additionally, the preparations from P. sativum and A. julibrissin hydrolysed albizziine to 2,3-diaminopropanoic acid, identified by co-chromatography and electrophoresís with authentic samples. It is concluded that there is a greater production and diversion of uracil into secondary product formation in the plants producing these compounds than in those that do not. In the non-producing plants, there is a more active catabolism of uracil. With A. julibrissin, degradation of 5-aminouracil by the pyrimidine catabolic process was shown to yield the non-protein amino acids albizziine and 2,3-diamino-propanoic acid. In Lathyrus tingitanus, this process demonstrably produces 4-hydroxyhomoarginine from lathyrine. The observations described are consistent with the view that production of pyrimidine-derived secondary products is attributable to the operation of detoxication processes for bioactive pyrimidines.