Vitamin B₆ Metabolism in McA-RH7777 Cells

Abstract

The metabolism of vitamin B₆ in McA-RH7777 cells has been characterized with respect to pyridoxal 5′-phosphate (PLP) levels, and the activities of pyridoxine (PN) kinase (EC 2.1.7.35) and pyridoxine 5′-phosphate (PNP) oxidase (EC 1.4.3.5). PLP levels (12.4 ± 4.4 ng/mg protein) were at the lower end of the range found for Morris hepatomas, carcinogen-induced rat hepatomas, and liver from rats fed a PN-deficient diet. PN kinase activity was about one-third of that found in normal rat liver. PNP oxidase appeared to be absent in high-speed supernatants of homogenates prepared from McA-RH7777 cells. The absence of PNP oxidase was supported by enzymatic and immunological data. These findings resemble those found previously for Morris hepatoma 7777. In contrast to rat liver, such preparations caused little or no release of volatile counts upon incubation with either [³H-C4′]PN or [³H-C4′]PNP. High-speed supernatants of homogenates prepared from both McA-RH7777 cells and Morris hepatoma 7777 were very much less capable than similar preparations from rat liver in converting [G-³H]PN to PLP and pyridoxamine 5′-phosphate. Despite the apparent absence of PNP oxidase, intact confluent or log-phase McA-RH7777 cells were capable of converting [G-³H]PN to PLP and pyridoxamine 5′-phosphate. These findings are discussed in terms of tumor nutrition and vitamin B₆ metabolism in a rat hepatoma cell line.