The
methionine cycle and its metabolites
homocysteine and
cysteine serve several important functions in cellular metabolism. Abnormalities in metabolism of the
methionine cycle have been associated with
cancer. We determined plasma levels of
methionine,
homocysteine and
cysteine in nude mice implanted with human
cancer cell lines (MDA-MB-435 breast, PC-3 prostate, HT29 colon, BX-PC3 pancreas) over a prolonged period of
tumor growth. The data were compared with correspondins values in nontumor-bearing controls. Nude mice were injected s.c. in the right flank with 10(6)
cancer cells.
Tumor growth was measured over time.
Methionine was measured in plasma by HPLC.
Cysteine and
homocysteine were measured in plasma by recombinant
enzyme assays and spectrophotometry to measure products. The concentrations of
cysteine and
homocysteine in plasma decreased significantly as a result of progression of breast, prostate and the pancreas
tumor types implanted in the nude mice at least over a two-month period. Data for the colon
tumors were nonsignificant for both
cysteine and
homocysteine. In the case of
methionine, the decrease was significant only due to progression of the
breast tumors, grown over a long time period, as compared to the mice without
tumors control. The results suggest that sulphur
amino acids may be plasma or serum
biomarkers for
cancer progression.