Inhibition of a model protease -: pyroglutamate aminopeptidase by a natural oligosaccharide gum from Hakea gibbosa

The purpose of this study was to investigate the effect of the oligosaccharide gum from Hakea gibbosa on the activity of a model protease enzyme pyroglutamate aminopeptidase (5-oxoprolyl peptidase; EC 3.4.19.3) and to elucidate the mechanism responsible for the decreased activity, Enzyme kinetic studies were conducted at 37 degreesC in 100 mM potassium phosphate buffer with 10 mM EDTA, 5% (v/v) glycerol, and 5 mM DTT (pH 8) for 15 min and were performed both in the presence and absence of the gum. Enzymatic activity was determined by a colorimetric assay using the specific substrate L-pyroglutamic acid beta -napthylamide. The enzyme kinetics was studied at various substrate and gum concentrations. The velocity of the reaction was determined by the amount of the product (beta -napthylamine) liberated at each substrate and gum concentration. The K-s and V-max of the enzyme in the absence of the gum were 24.40 +/- 2.14 muM and 502.95 +/- 28.90 nmoles.min(-1).mg protein (-1); respectively. As the concentration of the gum was gradually increased from 0.1 to 2%, the value of the V-max decreased from 318.94 +/- 21.46 to 158.83 +/- 24.51 nmoles.min(-1).mg protein(-1) while K-s increased from 17.42 +/- 4.6 to 63.03 +/- 1.89 muM. The mechanism for the inhibition of the-enzyme by Hakea appeared to be a mixed-linear type (a type of non-competitive inhibition) as suggested from Hanes-Woolf, Dixon and Cornish-Bowden plots. The turnover number, k(cat), calculated for the enzyme also decreased from 14.09 +/- 0.81 to 4.45 +/- 0.69 min-L as the concentration of the inhibitor was incrementally increased from 0 to 2%: (w/v). The K-i and alphaK(i) calculated from Dixon and Cornish-Bowden plots were found to be 0.31 +/- 0.11% (w/v) and 1.33 +/- 0.42% (w/v), respectively. The natural gum from Hakea gibbosa was effective in non-competitively inhibiting the enzyme pyroglutamate aminopeptidase. Thus, the natural gum may be a promising additive not only for its sustained-release and mucoadhesive properties as shown previously, but also for its ability to slow the enzymatic degradation of therapeutic polypeptides incorporated in dosage forms. (C) 2001 Elsevier Science B.V. All rights reserved.