Enzymatic formation of the pteridine moiety of folic acid from guanine compounds.

drosylation reaction and may account for the observed inhibition when crude sepia extracts are tested at high concentrations. The K, for sepia pteridine in the hydrosylation reaction is 3 to 4 X 10e6 M. On a molar basis it is approximately 25 times more active than tetrahydrofolate. The maximal rate of tyrosine formation at saturating levels of cofactor is about the same for sepia pteridinc and for the rat liver compound. The structural relationship between sepia pteridine and the rat liver cofactor is not known. Indeed, there is still some uncertainty about the structure of sepia pteridine. IVawa (9) has proposed the structure shown in Fig. I-4, and that favored by Viscontini and Miihlmann (15) is shown in Fig. 1B. .%ccording to both formulations, sepia pteridinc is closely related to biopterin,4 which also occurs in the insect cyc (5, 7). The report by Taira (18) that sepia pteridine is a substrate for dihydrofolic reductase (19, 20), an enzyme bclievcd to bc specific for 7,8-dihydroptcridines (20), would seem to bc more consistent with the structure shown in Fig. 1A. Recently we have found that, in addition to the purified rat and sheep liver enzyme, the activity of sepia pteridine and the purified liver cofactor in the hydrosylation system is dependent on dihydrofolic reductase. This result indirectly c,onfirms Taira’s finding with respect to sepia pteridinc and furthermore provides strong support for the idea, mentioned earlier, that the liver cofactor is a dihydropteridinc. Moreover, if one makrs the reasonable assumption that dihydrofolic reductase shows the same pteridine specificity as the folic reductasc system studied by Zakrzewski (21), one can conclude further that the purified rat liver cofactor is a 2-amino-4-hydrosy-7,8-dihydropt~idine~