Metabolic actions of middle molecules.

About 20 inhibitory activities of middle molecular range were described and characterized sufficiently. The most useful classification divides them on the basis of their mobility on a column of Sephadex G-15. This classification enables us to compare the analytical and preparative data, the results of various laboratories and to suggest identity of various inhibitors. It is useful to outline the localization of the inhibitor action at the cellular level, to define the type of inhibitory activity and eventually to identify the second messenger of the action of inhibitor. The measurement of middle molecular (MM) fractions with various sophisticated methods has clearly established that there is a number of MM substances accumulating in biological fluids of patients with chronic renal failure. A part of them is probably of little clinical significance. However, those with biological activity may be of great clinical implication. To omit the interference of biologically inactive MM substances and to define the biological activity of those of clinical relevance the search turned to be biological actions of MM substances during the last years. The alteration such as impaired glucose utilization is chosen on the basis of clinical suggestion and the responsible inhibitor is looked for. This approach reflects in completely different strategy in comparison with the analytical search. Preparative instead of analytical arrangement is to be used. This does not mean only volume differences. The principles differ. This is to be stated to prevent inappropriate loss of time and effort of those starting to search in this manner.