Radiation inactivation of oligomeric enzyme systems: theoretical considerations.

Abstract

Radiation inactivation is used as a tool to explore the size, structure and function of soluble and membrane-bound enzymes. Enzyme systems consisting of a single functional unit yield a single exponential dependence of enzyme activity on radiation dose. Complex, nonexponential, inactivation curves suggest the presence of a multiunit enzyme system. A concave-upward inactivation curve suggests the presence of multiple independent functional units of distinct size and activity that do not interact with each other. An oligomeric enzyme, consisting of n identical subunits in equilibrium with monomers, can give simple exponential decay curves or more complex inactivation curves with various degrees of upward or downward concavity; this depends upon the extent of oligomer-monomer equilibration among subunits after radiation, oligomer-to-monomer size and activity ratios, and multihit requirements for oligomer inactivation. For each of these possibilities, equations for the inactivation curves are derived, calculated numerically and discussed in qualitative terms. A systematic approach to the evaluation of complex radiation inactivation curves is proposed and limitations of the radiation inactivation method are reviewed.