THE INCORPORATION OF 14C-THREONINE AND 14C-GLUCOSAMINE INTO SUBCELLULAR FRACTIONS AND INTO BOVINE SUBMAXILLARY MUCIN BY SLICES OF BOVINE SUBMAXILLARY GLAND

Bovine submaxillary gland slices were used to investigate the subcellular site of biosynthesis of bovine submaxillary mucin (BSM). Slices were incubated with 14C-threonine and 14C-glucosamine in Krebs medium III at pH 7.4 and 37 °C for varying periods of time up to 180 minutes. Methods of subcellular fractionation based on those developed for liver proved unsatisfactory for use with submaxillary gland, probably because of the presence of large amounts of highly viscous mucin. A method of subcellular fractionation was developed which yielded five fractions: P (total soluble protein) and C (soluble BSM, purified by Cetavlon precipitation) were prepared from the 16,000 × g supernatant; and R (ribosomes), S (deoxycholate-soluble membrane-bound protein), and a residual fraction D were prepared from the 16,000 × g pellet. The incorporation of both tracers into these fractions was time-dependent and was inhibited by preincubation of the slices for 15 minutes with puromycin. The highest specific activities were r...