Non-enzymic protein induced hydrolysis of P-nitrophenyl acyl esters in relation to lipase/esterase assays

Protein-induced hydrolysis ofP-nitrophenyl acyl esters has been studied mainly via the reaction between bovine serum albumin (BSA) and P-nitrophenyl myristate. In the present study BSA caused a large increase in the amount of P-nitrophenol released (measured as increase in absorbance at 410 nm) at pH above 6.5, but at lower pH BSA had only negligible catalytic activity. BSA-catalysed hydrolysis of P-nitrophenyl acyl esters increased with temperature (25–80 °C), concentration of BSA and concentration of P-nitrophenyl acetate, but was virtually unaffected by changes in P-nitrophenyl myristate concentration. The rate of hydrolysis was highest for P-nitrophenyl acyl esters with an acyl moiety of 8–12 carbon atoms at a p-nitrophenyl concentration (0.028 mmol/litre) below critical micelle concentration (CMC) for all p-nitrophenyl acyl esters tested. For a p-nitrophenyl acyl ester concentration (0.28 mmol/litre) above CMC (acyl moiety of ≥6 carbon atoms) the rate of hydrolysis decreased with increasing chain length of the acyl moiety. The catalytic effect of BSA decreased with heat treatment (90–95 °C for 15 min) of the protein solution, but did not disappear. Tween-20 effectively retarded BSA-catalysed hydrolysis of p-nitrophenyl myristate, and the effect was dependent on both Tween-20 and BSA concentrations. Free fatty acids displayed a similar but less pronounced inhibitory effect on the hydrolytic reaction. The inhibitory effect increased with the length of the fatty acids (C-4 to C-10).