Influence of liposomes on tryptic digestion of insulin. II

The mechanism of enhancement of tryptic digestion of insulin by empty liposomes was studied using HPLC analysis, gel filtration (insulin binding to the liposome and lipid transfer to the insulin) and an electrokinetic study using the zeta meter (trypsin binding to the liposome). Soybean phosphatidylcholine (PC), phosphatidic acid (PA) [PA/PC = 1/9] and stearyl amine (StA) [StA/PC = 1/9] were used as neutral, negatively charged and positively charged liposomes, respectively. Tryptic digestion of insulin was enhanced by reducing the liposome size from 150 to 40 nm when neutral empty liposomes were used. The amount of insulin bound to neutral empty liposomes increased on reducing liposome size. Nevertheless, no strong evidence of trypsin binding to neutral empty liposomes was obtained. The amount of liposome-bound insulin was greater for PC than StA/PC and PA/PC, and the amount of lipids transferred to insulin decreased in the following order; StA/PC > PA/PC > PC. These findings suggest that the positively charged liposome did not enhance tryptic digestion, because insulin was protected from tryptic digestion by surrounding lipids from positively charged liposomes (StA/PC). Trypsin bound to the PA/PC liposomes, but not to the PC or StA/PC liposomes.