Comparative Properties of Three Pteridine Reductases

Naturally occurring pteridines in eukaryotic systems usually contain 2-amino, 4-hydroxy 6-alkyl substituents composed of either a methylene-p-aminobenzoylglutamate (or polutamate) or a dihydroxypropyl group. The former class are known collectively as the folates and occur widely as reduced and 5-alkylated derivatives, in which form they participate in important metabolic one-carbon transfers (Blakley, 1984). The latter, known as biopterin,, also occurs in reduced forms and is an important cofactor in aromatic amino acid hydroxylations en route to the catecholamines (Shiman, 1985; Kaufman and Kaufman, 1985; n and Lovenberg, 1985) and in the nitrite synthase pathway (Marletta,, 1993). Dihydrofolate reductase (DHFR), dihydropteridine reductase (DHPR) and pteridine reductase (PTR1), best characterised from Leishmania, are three enzymes that initiate the reduction of a pteridine in association with a reduced dinucleotide cofactor. Their comparative reaction pathways are illustrated in Figure 1.