Elevated Methionine-tRNA Synthetase Activity in Human Colon Cancer 1

Summary

Methionine-tRNA synthetase catalyzes the formation of methionine-tRNA (Met-tRNA) which is necessary for the initiation of protein biosynthesis. This study compares Met-tRNA synthetase activity in extracts of human colon cancer and normal colonic mucosa.

Particle-free supernatants were prepared from homogenates of tumors and adjacent normal mucosa. These preparations were incubated with ATP, a mixture of yeast tRNAs, Mg²⁺, K⁺, dithioerythritol, and [³H]methionine. Specific activities (nano-moles of [³H]Met-tRNA per milligram of protein) of Met-tRNA synthetase from tumors and normal mucosa were measured and compared.

Five patients were studied and in each the tumor sample had a higher specific activity than its normal counterpart. The mean specific activity of Met-tRNA synthetase in the tumor tissue was approximately four times that of the normal mucosa. Methioninyla-denylate, when added to the assay tubes, proved to be a potent inhibitor of Met-tRNA synthetase.

Since protein biosynthesis begins with the formation of Met-tRNA, the synthetase may be a rate-limiting enzyme governing protein biosynthesis in tissues. High specific activity of the enzyme would be expected to correlate with rapid cell growth. Inhibition of the enzyme with methioninyladenylate may prove a useful method of regulating protein biosynthesis in tumor cells.

The authors express their appreciation to Sylvia Buorge-Berman for technical assistance and to Drs. Leroy Townsend and Arthur Lewis for the synthesis of methioninyladenylate.