Classification of difference between inhibition constants of an inhibitor to facilitate identifying the inhibition type.

To identify the common inhibition types, the putative decision system is unsatisfactory. In a new decision system, Michaelis-Menten constants and maximal reaction rates were plotted versus inhibitor concentrations for deriving K(ik) and K(iv) as the inhibition constants, respectively; their difference was quantified as the ratio of the larger one to the smaller one. Such ratios below 2.0 suggested uncompetitive inhibitors, over 5.0 suggested noncompetitive or competitive inhibitors, and from 2.0 to 5.0 suggested mixed inhibitors. By the new decision system, (i) the simulation recovery of uncompetitive inhibitors under CVs of 2% or 5% was improved by four times, but that of competitive or noncompetitive inhibitors was improved slightly; (ii) the recovery of L-phenylalanine as an uncompetitive inhibitor of intestinal alkaline phosphatase reached 38%, while the putative decision system lost all; the recovery of xanthine as a competitive inhibitor of uricase was improved slightly. Therefore, the new decision system was better.

An integration strategy to estimate the initial rates of enzyme reactions with much expanded linear ranges using uricases as models.

A new strategy was proposed to estimate the initial rates of reactions catalyzed by Michaelis-Menten enzymes via integrating the classical initial rate method for low activities with an improved integrated method for high activities. Between these two individual methods, this integration strategy required: (a) the consistent linear response slopes, acquired with an optimized preset substrate concentration (PSC) to derive the initial rates from the maximal reaction rates estimated by the improved integrated method; (b) an overlapped region of the initial rates measurable with consistent results, realized with an optimized reaction duration to record reaction curves for analyses by the improved integrated method; (c) a switch cutoff, preset as the instantaneous substrate concentration slightly above that after a given lag time when the enzyme activity was just below the upper limit for the linear response of the classical initial rates. By simulation with uricases at a given initial substrate concentration (S(0)), the optimized PSC was 93% S(0), the optimized reaction duration at S(0) from 0.35-fold to 11.0-fold Michaelis-Menten constant (K(m)) was within 6.0 min and the switch cutoff was available at the given S(0) after 30-s lag time, all of which were combined to produce 300-fold linear ranges. By experimentation with one uricase of K(m) at 6.6 microM and the other uricase of K(m) at 220 microM under optimized conditions, this integration strategy with S(0) at 75 microM produced 100-fold linear ranges. Thus, this integration strategy exhibited much expanded linear ranges and practical efficiency over wide ratios between S(0) and K(m).