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to challenge that there is a separate site for valine. (allomfree) was purchased from Sigma The so-called abnormal kinetic properties of c~~~~~~. threonine deaminase are best shown in the sub- The preparation of crude cell extract from Eschevichia strate saturation curves which are sigmidaj coki 9723 which was used as the source of threvnine rather than hyperbolic. H~~~~~~, this unusual deaminase has been described in detail (5). The general assay method has been described (5). Modifications of response of the enzyme to varied concentrations ,he Benerd method are in the figures. of substrate has been shown to be due to a remarkable degree of instability of threonine Results deaminase (after dilution for a test) in low con- Stabjljzarjon $Dilute Enzyme by Valine or centrations of substrate (45). The diluted holeucine enzyme is increasingly stabilized as the concentra- 1, the experiments described in ~i~, 1,20 m~ tion of the substrate is increased. When L-threonine was usede ~h~ enzyme was conC~lTeCti~ns are made for loss of enzyme during sidered to be stabilized by this concentration the test, the curve is hyperbolic When since the rate of reaction was shown not to stabilization is accomplished by high buffer decrease for 25 min. H~~~~~~, the diluted concentration* the curve is hyperbolic also (5)- enzyme is so extremely unstable in 0.1 M potasSince the "abnormal9' kinetic properties of sium phosphate buffer in the absence of threonine threonine deaminase are due to instability of the (5) that significant errors may be introduced enzyme at low thonine concentration, and, unless appropriate precautions are exercised. since valine "normalizes" the kinetics by "activa- ~h, enzyme was stored at - 150 in 0.5 M potastion" at low threonine concentration, it seems sium phosphate (chloride-free) buEer (p~ 8-21. when a test was completely prepared, a portion |
