Advances in Cancer Research 10. [antikvár]

therefore, important to summarize briefly somé of the factors which may affect the observed activity of an enzyme system. Enzyme profiles of animal tissues are generally obtained by the use of preparations of tissue slices, homogenates, subcellular fractions, or extracts. Any given enzyme activity is usually determined in the presence of what are believed to be optimál levels of the required substrates and cofactors. In crude tissue preparations, however, there are a multiplicity of factors which influence the observed activity and which frequently are not or cannot be controlled. Since the majority of citations in this review refer to work in which crude preparations were used, one should be aware of the largely operationaj nature of the description of the enzyme content of a given tissue. Although the use of unpurified preparations may have limitations in describing the enzyme profilé of a tissue the method of purification and isolation of specific enzymes from tissues alsó has its shortcomings when this information is used to extrapolate to the functional in vivo content of enzyme. Thus, enzymes may be activated or inactivated either reversibly or irreversibly during their isolation and purification. In addition, it is evident that both approaches, i.e., the assay of crude preparations and the isolation of purified enzymes, involve a destruction of cell architecture and normál intracellular or intercellular control mechanisms involving a host of factors such as pH, ionic strength, substrate, cofactor, and product localization, all of which may affect enzyme activity. Enzyme control mechanisms may be classified into two broad categories. First, there are those factors that affect the enzymatic activity of specific protein molecules, and second, those factors that regulate the amount of enzyme protein. In respect to the former, the activity of an enzyme may be affected by the concentration of either substrate or product. A prime example of substrate inhibition is the inhibitory effect of high levels of acetylcholine on acetylcholinesterase (Nachmanson and Wilson, 1945). Typical product inhibition is exemplified by the inhibition of hexokinase by its reaction product glucose-6-phosphate (Crane and Sols, 1953). The latter may be an example of an "isosteric inhibition." The inhibitor is sterically related to the substrate or cofactor and competes with either of the latter for the catalytic site on the enzyme. Thus, isosteric inhibition may take place if the tissue preparation contains a compound which competes with substrate or cofactor