PRECIPITATION OF HEVEA-BRASILIENSIS LATEX PROTEINS WITH TRICHLOROACETIC-ACID AND PHOSPHOTUNGSTIC ACID IN PREPARATION FOR THE LOWRY PROTEIN ASSAY

Many proteins derived from the latex of Hevea brasiliensis that remain soluble in trichloroacetic acid (TCA) can be precipitated by phosphot ungstic acid (PTA). A combination of 5% TCA and 0.2% PTA precipitates a wide range of proteins effectively even when they are present in low concentrations (below 1 mu g ml(-1)). In addition to its protein puri fication function, acid precipitation also increases the sensitivity o f the subsequent protein assay by allowing the test sample to be conce ntrated, Another advantage of protein precipitation by TCA and PTA is that very small amounts of protein (of the order of 10 mu g) can be re peatably recovered without the use of precipitate-bulking agents such as sodium deoxycholate. This general procedure of protein purification and concentration is simple and rapid, but the use of PTA may not be fully compatible with the Bradford protein assay, A modified Lowry mic roassay is described which enables about 3 mu g ml(-1) to be quantitat ed at the photometric absorbance of 0.05. When used in conjunction wit h protein concentration by precipitating with TCA/PTA, approximately 0 .4 pg ml(-1) protein present in 6 mi of solution can be assayed. (C) 1 995 Academic Press, Inc.