Often the preparative steps in a tissue analysis are the most critical. In the case of a metabolite assay, the most hazardous period is usually between the moment the blood supply is cut off and the moment enzyme action is finally stopped. In the case of an enzyme, with a few important exceptions, there is not likely to be much change for many minutes or even hours. Instead, the biggest problem may be to render the enzyme fully accessible for assay, or to prevent loss during or after homogenization. In a few cases, if the objective is to determine the state of activity as it was in vivo, the problem may be to prevent a specific activity change of the enzyme (e.g., the conversion of phosphorylase b to a, and so on).