DETERMINATION OF PROTEIN-SYNTHESIS IN HUMAN RECTAL-CANCER IN-SITU BY CONTINUOUS [1-C-13]LEUCINE INFUSION

Previous studies on human colorectal tumor protein synthesis in situ r elied on techniques that required intra- or perioperative sampling to obtain a sufficient biopsy size. The purpose of the present study was to develop a new technique by use of new mass spectrometry equipment [ capillary gas chromatography (GC)-combustion isotope ratio mass spectr ometry (IRMS)], which allows reduction of the necessary sampling size. Thereby, tumor sampling could be done via conventional rectosigmoidos copy, excluding the need for further disturbing invasive measures. Fif teen postabsorptive patients with localized rectal cancer received a p rimed-constant infusion of [1-C-13]leucine (0.16 mu mol.kg(-1).min(-1) constant, 9.6 mu mol/kg prime). Forceps biopsies were taken after 3 a nd 6 h. In five subjects, tumor tissue and normal mucosa were studied simultaneously. Determination of protein-bound leucine enrichment was done by GC-IRMS, and GC-quadrupole MS was used to determine tracer-to- tracee ratios (tracer/tracee) for free intracellular leucine. CC-MS da ta demonstrated achievement of a steady state in the precursor pool en richment after 3 h of isotope infusion (tracer/tracee at 3 h: 6.34 +/- 0.46%, at 6 h: 6.58 +/- 0.38%). Calculation of tumor protein synthesi s yielded a fractional synthetic rate (FSR) of 1.06 +/- 0.11%/h or 25. 5 +/- 2.6%/day (range 12.0-37.1%/day). At any time, protein-bound leuc ine enrichment was significantly higher in tumor tissue than in normal mucosa of the same subject. However, protein synthetic rates were com parable (tumor: 1.09 +/- 0.20%/h, mucosa: 1.29 +/- 0.28%/h). Thus comb ined CC-combustion IRMS and GC-/quadrupole MS provide a simple, reliab le, and minimally invasive method to determine tumor FSR in situ, ther eby excluding interferences common to previous methods. Tumor and muco sa tissues are similar with respect to protein synthesis, but they app arently differ with respect to leucine extraction from the arterial bl ood.