Characterization of the pools of 5,10-methylenetetrahydrofolates and tetrahydrofolates in xenografts of human colon adenocarcinoma.

The method for measuring polyglutamate forms of CH2-H4PteGlu and H4PteGlu, by entrapment in ternary complexes with [6-3H]5-fluoro-2'-deoxyuridylate and Lactobacillus casei thymidylate synthase has been characterized. Results demonstrated that (a) the relationship between concentration of CH2-H4PteGlu and complex isolated on nondenaturing gels was dependent upon the number of glutamyl residues, and an alternative method for data analysis has been presented, (b) the relationship was linear over a 100-fold change in concentration, (c) formation of isolatable complex was time dependent, (d) noncovalent complexes formed with PteGlu2-5 could be isolated only at concentrations considerably higher than those required for CH2-H4PteGlu1-6, and (e) endogenous deoxyuridylate would be unlikely to interfere significantly with the assay. The distribution of polyglutamates of CH2-H4PteGlu and the combined pools of CH2-H4PteGlu plus H4PteGlu were subsequently examined in three human colon adenocarcinoma xenografts. In each tumor, the pentaglutamate of CH2-H4PteGlu and H4PteGlu was the most prominent species, followed by the hexaglutamate, constituting 68 to 92% of the CH2-H4PteGlu pool, and greater than 93% of the combined pools. A small percentage of di-, tri-, and tetraglutamates were also detected. Using a catalytic assay, the combined pool of CH2-H4PteGlu and H4PteGlu was estimated in the range of 0.5 to 2.7 microM in cell water, and for CH2-H4PteGlu, from 185 nM to 1.7 microM. Using thymidylate synthase purified from colon adenocarcinoma HxVRC5, CH2-H4PteGlu5 (where the subscript digit attached to the glutamate portion equals the number of glutamate residues) stabilized the covalent ternary complex at greater than 200-fold lower concentration in comparison to CH2-H4PteGlu1. Data indicated that in each colon tumor, the concentrations of CH2-H4PteGlun or CH2-H4PteGlun plus H4PteGlun were suboptimal for the interaction of 5-fluoro-2'-deoxyuridylate with thymidylate synthase, and would predict for relatively transient inhibition of thymidylate synthase after treatment with 5-fluorouracil. These data support therapeutic modulation to increase the concentration of CH2-H4PteGlun in the treatment of colon adenocarcinomas with 5-fluorouracil.