1. A method is described for measuring the rates of protein synthesis in vivo in human colorectal and breast tumours by the intravenous injection of l-[1-13C]leucine as a ‘flooding dose'.
2. The incorporation of isotope into colorectal tumour protein was measured in six patients, whose tumours were biopsied after the injection. Fractional rates of protein synthesis were calculated from the enrichment of leucine in protein and the average free leucine enrichment in plasma. The range of rates obtained was 17.2–33.9%/day, with a mean rate (± sem) of 22.5 ± 2.6%/day.
3. Tumour protein synthesis rates were also measured in 15 patients with breast cancer. The range of rates obtained was 5.3–15.9%/day, with a mean rate (± sem) of 10.3 ± 0.8%/day. These rates are significantly lower than those obtained with colorectal tumours (P < 0.001).
4. In 9 of the breast cancer patients, protein synthesis was measured in multiple random biopsies taken from the same tumour. The mean (± sem) difference between the highest and lowest rates in biopsies from the same tumour was only 1.1 ± 0.3%/day. Only 13% of the variation in protein synthesis between separate tumours could be explained by sampling error because of variation within the tumour itself, the remainder being genuine variation between individual tumours.