Enzyme Activity in Serum or Plasma

Since this symposium is concerned with enzymes in medicine, I shall consider only the work that has been done in human malnutrition. It would be impossible to deal with the very large literature on enzyme changes in animals under different dietary conditions. I shall also confine myself to work which is related to protein and calorie deficiency because this is the field with which I am familiar, although since most of the B-complex vitamins act as cofactors for one or more enzymes of intermediary metabolism or of the respiratory chain, certain enzyme measurements may be of value in the diagnosis of vitamin deficiencies, eg, the measurement of red-cell transketolase (EC 2.2.1.1.) in thiamine deficiency and of glutathione reductase (EC 1.6.4.2.) in riboflavin deficiency. Studies on enzyme changes in human subjects with protein-calorie malnutrition have, broadly speaking, followed three paths, with different aims and different underlying assumptions. These may be summarized as follows: (1) the study of enzymes as 'protein markers' to give information about the functional state of protein metabolism in the body as a whole, or in one tissue or organ; (2) the search for increases in enzyme concentration in blood or urine as evidence of cell damage; (3) the study of enzyme changes in tissues, in order to identify the underlying biochemical lesion in any given state of malnutrition.