Hepatic parenchymal cell glutamylation of methotrexate studied in monolayer culture.

A detailed analysis of the gamma-glutamylation of methotrexate has been conducted in primary cultures of rat hepatic parenchymal cells in monolayer culture. The rates of glutamylation are concentration dependent and saturable when measured over a 6-h period at concentrations between 2 and 50 microM. The removal of folate and inclusion of insulin, dexamethasone, and tocopherol enhance glutamylation. Omission of methionine from the medium increases glutamylation, whereas the 6-h period of syntheses, methotrexate the reaction. During the 6-h period of syntheses, methotrexate diglutamate is the primary product, whereas the di- and triglutamates are the major cellular species when the incubation is extended to 24 h. Lower extracellular methotrexate concentrations result in the formation of relatively greater amounts of longer chain length derivatives. The accumulation of methotrexate polyglutamates at steady state is saturable and occurs by 24 h. The predominant species contain two to four glutamate residues, and the distribution depends upon the culture conditions and the extracellular methotrexate concentration. The turnover of cellular polyglutamates at saturation occurs at 30 to 40% of the total cell pool over a 6-h period. Placement of hepatocytes with saturating levels of methotrexate polyglutamates in medium lacking methotrexate results in the slow loss of all derivatives, and the rate of loss is inversely related to polyglutamate chain length. Following a pulse dose of methotrexate, hepatocytes continue to increase the chain length of the cellular pool of polyglutamates, and this process is impaired by addition of folinic acid to the medium. In the pulse experiments as in the longer term incubations, the primary species are the di- and triglutamates. The results demonstrate limited capacity of the hepatocytes to glutamylate longer chain length polyglutamate derivatives.