Enzymes of the Human Erythrocyte

A detailed description of appropriate procedures for the isolation in presumably ahnost pure form of a purine nucleoside phosphorylase from human erythrocytes was presented as the original report of Paper I (1). The present paper will s ummarize the results of investigations concerning the stability characteristics and kinetic properties of the isolated enzyme. Enzymatic degradation of nucleoside leading to liberation of free purine was described in the early literature (2, 3); however, the phosphorolytic (and arsenolytic) nature of this reaction was not established prior to the more recent investigations by Kalckar (4, 5). Phosphorolysis of purine ribosides and the reversible nature of this reaction were initially demonstrated for inosine and guanosine with use of a purified rat liver preparation (5). Subsequently, it has been shown that liver preparations were similarly active on purine deoxyribosides (6), xanthosine riboside at high enzyme concentration (7), and nicotinamide riboside (8) (riboside synthesis with S-azaguanine (9) as well as 2,6-diaminopurine, adenine, and 4-amino5-imidazole carboxamide (10) was also reported). In addition to phosphorolytic cleavage of nucleoside, a simple hydrolytic mechanism haa also been recently demonstrated with a yeast preparation (11). A similar mechanism has not yet been shown with mammalian tissues.