THE ALTERATION OF INTRACELLULAR ENZYMES

Abstract

1. The very large increase in catalase activity (Euler effect) which follows treatment of yeast cells with CHCl₃, UV and n-propanol is accompanied by highly significant changes in kinetic properties. With respect to the enzymatic decomposition of H₂O₂, the thermodynamic constants of the activation process µ, ΔH‡, ΔS‡, ΔF‡, decrease, following treatment of the intracellular enzyme, by 4.5 kcal., 4.5 kcal., 10.1 e.u. and 1.7 kcal., respectively, all these differences being significant at the 1 per cent level. 2. Similar differences exist between the untreated, intracellular enzyme on the one hand, and the extracted yeast and crystalline beef liver catalases on the other. Significant differences in these thermodynamic constants do not exist among the treated intracellular, extracted yeast, and crystalline liver catalases. 3. These data provide unequivocal confirmation of the phenomenon of enzyme alteration reported previously, and confirm previous evidence that the extracted and crystalline enzymes have also undergone enzyme alteration and have properties which are identical with, or very similar to, those of the catalase altered in situ. 4. With respect to the process of heat destruction of catalase, the greatly diminished stability to heat of the altered enzymes, previously reported, has been confirmed. The thermodynamic constants of activation of this process have likewise changed following alteration, in the case of µ, ΔH‡, and ΔS‡ an increase of 20.6 kcal., 20.6 kcal., and 70 e.u., respectively, and of ΔF‡ a decrease of 2.8 kcal. 5. All these data have been shown to be consistent with, and in some cases predictable from, the interfacial hypothesis, which states that the unaltered catalase exists within the cell adsorbed to some interface, in a partially, but reversibly, unfolded configuration of relatively low specificity; enzyme alteration consists, in the case of catalase, of desorbing the enzyme from the interface into its rolled-up, soluble, highly specific configuration. While the interfacial hypothesis has successfully withstood this experimental attack, the present data do not provide its unequivocal proof, since they are consistent with any hypothesis of alteration in which the unaltered, intracellular enzyme is in a relatively disordered state by comparison to the altered enzyme. While evidence of an interfacial process in enzyme alteration has been adduced previously, critical proof of the interfacial hypothesis awaits creation of a model system, in which most of the aspects of intracellular alteration can be reproduced. 6. Certain of the changes in kinetic properties following alteration of the intracellular enzyme, such as increased activity and the modified energies and entropies of activation of both enzyme-substrate system and heat destruction of the catalase itself, might be explained by a decrease (two orders of magnitude) in the effective hydrogen ion concentration, allowing the intracellular enzyme to be brought to the same pH as the extracellular medium. If such a pH change does, in fact, occur, it is necessary to invoke the interfacial hypothesis to explain why the unaltered, intracellular enzyme is in equilibrium with a medium whose pH is approximately 2 units lower than that of the cytoplasm itself. 7. It is concluded that kinetic data of this kind may be used to shed light on the structure of a soluble, cytoplasmic enzyme, not attached to any of the formed elements within the cell, yet organized within it in a condition of relatively low structural specificity; further, that information obtained exclusively from a study of the kinetics of the extracted or crystalline enzymes may not, in the case of this enzyme, at least, be extrapolated to the same enzyme within the intact cell.

Full Text

The Full Text of this article is available as a PDF (1.7 MB).