Читать книгу Plant Nucleotide Metabolism - Hiroshi Ashihara - Страница 72

As noted above, in most prokaryotes and nearly all eukaryotes, HGPRT catalyses the salvage of hypoxanthine and guanine. However, in a few species, xanthine salvage enzyme activity is also detected. Guanine and xanthine phosphoribosyltransferase (GXPRT, EC 2.4.2.22) activity has been observed in Lactobacillus casei and E. coli (Krenitsky et al. 1970). More specific xanthine phosphoribosyltransferase (XPRT) is found in Leishmania donovani, a protozoan parasite. This enzyme preferentially uses xanthine; the Km value for xanthine (7 μM) is much lower than for hypoxanthine and guanine (100–450 μM) (Jardim et al. 1999). In plants, low phosphoribosylation of xanthine was detected in tea leaf extracts in vitro (Deng and Ashihara 2010). However, it is unclear whether this activity is due to a distinct XPRT or if it is a side reaction of HGPRT (Ashihara et al. 2018).