L-Histidinol dihydrochloride

The effects of L-histidinol on the susceptibility of the transplantable murine mast-cell neoplasm P815 mastocytoma to selected anticancer drugs have been evaluated on cells growing in culture and in syngeneic DBA/2J mice. Combinations of L-histidinol and anticancer drugs of either phase specificity [cytarabine (ara-C) and vinblastine sulfate] or cycle specificity [5-fluorouracil (FUra) and methotrexate] had diverse effects on cultured mastocytoma cells as scored by clonogenic cell survival assays. Flow cytometric analysis of randomly proliferating P815 mastocytoma cells revealed that although exposure to L-histidinol did not preclude cells from traversing the cell cycle, the analogue nonetheless conferred a dose-dependent and apparently nonspecific delay of cell cycle transit. DBA/2J mice bearing intraperitoneal P815 mastocytoma cells were used to evaluate the in vivo efficacy of L-histidinol-ara-C and of L-histidinol-FUra combinations. Quantitative cell survival assays of murine bone marrow cells and of clonogenic tumor cells obtained from treated animals demonstrated that L-histidinol eliminated the bone marrow toxicity otherwise attending the use of the drugs ara-C and FUra. Simultaneously, the inclusion of L-histidinol provided a statistically significant increase in the capacity of these two anticancer drugs to eradicate intraperitoneal mastocytoma cells.

L-Histidinol phosphate phosphatase (HPP) catalyzes the hydrolysis of L-histidinol phosphate to L-histidinol and inorganic phosphate, the penultimate step in the biosynthesis of L-histidine. HPP from the polymerase and histidinol phosphatase (PHP) family of proteins possesses a trinuclear active site and a distorted (β/α)(7)-barrel protein fold. This group of enzymes is closely related to the amidohydrolase superfamily of enzymes. The mechanism of phosphomonoester bond hydrolysis by the PHP family of HPP enzymes was addressed. Recombinant HPP from Lactococcus lactis subsp. lactis that was expressed in Escherichia coli contained a mixture of iron and zinc in the active site and had a catalytic efficiency of ~10(3) M(-1) s(-1). Expression of the protein under iron-free conditions resulted in the production of an enzyme with a 2 order of magnitude improvement in catalytic efficiency and a mixture of zinc and manganese in the active site. Solvent isotope and viscosity effects demonstrated that proton transfer steps and product dissociation steps are not rate-limiting. X-ray structures of HPP were determined with sulfate, L-histidinol phosphate, and a complex of L-histidinol and arsenate bound in the active site. These crystal structures and the catalytic properties of variants were used to identify the structural elements required for catalysis and substrate recognition by the HPP family of enzymes within the amidohydrolase superfamily.

The amino acid analogue L-histidinol reverses the multidrug-resistance (MDR) attribute of the colchicine-resistant (CHR) variant CHRC5, a Chinese hamster ovary cell line that overexpresses a plasma membrane-associated glycoprotein and is resistant to colchicine (CH), daunorubicin, and vinblastine sulfate (VS). The level of cell kill achieved in CHRC5 cells by combinations of L-histidinol and either daunorubicin or CH approached that achieved in AUXB1 parent cells by these two drugs, whereas L-histidinol-VS combinations killed even more CHRC5 cells than VS in the parental line. The capacity of L-histidinol to reverse the MDR phenotype of the CHRC5 line was time and dose dependent and was eliminated by the addition of a twofold molar excess of L-histidine. The reversal of the MDR trait by L-histidinol appears to be independent of the drug uptake mechanism.