Pyridoxal (PL) reductase encoded by the plr1+ gene practically catalyzes the irreversible reduction of PL by NADPH to form pyridoxine (PN). The enzyme has been suggested to be involved in the salvage synthesis of pyridoxal 5′-phosphate (PLP), a coenzyme form of vitamin B6, or the excretion of PL as PN from yeast cells. In this study, a PL reductase-disrupted (plr1Δ) strain was constructed and its phenotype was examined. The plr1Δ cells showed almost the same growth curve as that of wild-type cells in YNB and EMM media. In EMM, the plr1Δ strain became flocculent at the late stationary phase for an unknown reason. The plr1Δ cells showed low but measurable PL reductase activity catalyzed by some other protein(s) than the enzyme encoded by the plr1+ gene, which maintained the flow of "PL → PN → PNP → PLP" in the salvage synthesis of PLP. The total vitamin B6 and pyridoxamine 5′-phosphate contents in the plr1Δ cells were significantly lower than those in the wild-type ones. The percentages of the PLP amount as to the other vitamin B6 compounds were similar in the two cell types. The amount of PL in the culture medium of the disruptant was significantly higher than that in the wild-type. In contrast, PN was much higher in the latter than the former. The plr1Δ cells accumulated a 6.1-fold higher amount of PL than the wild-type ones when they were incubated with PL. The results showed that PL reductase encoded by the plr1+ gene is involved in the excretion of PL after reducing it to PN, and may not participate in the salvage pathway for PLP synthesis.
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