De Novo Purine Biosynthesis in Intact Cells of Cucurbita pepo

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Abstract
The capacity of intact cells of roots excised from summer squash plants to synthesize purine nucleotides de novo was investigated. The incorporation of [1-14C]glycine, [2-14C]glycine. NaH14CO3, and H14COONa into total adenine nucleotides was demonstrated. The addition of azaserine or aminopterin, known inhibitors of de novo purine synthesis in other organisms, blocked the incorporation of these precursors into adenine nucleotides. The purine ring synthesized from these precursors was labeled in a manner consistent with the pathway for de novo purine biosynthesis found in microorganisms and animal tissues. Under optimal conditions, the activity of this pathway in roots excised from 2-day-old squash plants was 244 .+-. 13 nmol (mean .+-. standard error, n = 17) NaH14CO3 incorporated into .SIGMA.Ade (the sum of the adenine nucleotides, nucleoside and free base) per gram tissue during the 3-h incubation period. The possible occurrence of alternative enzymic reactions for the 1st steps of de novo purine biosynthesis was also investigated. No conclusive evidence was obtained to support operation of alternative enzymic reactions in the intact cell of C. pepo.