STUDIES OF THE INSULIN-LIKE ACTIVITY OF SERUM

Abstract

ABSTRACT The insulin-like activity (ILA) of serum and serum protein fractions was investigated by the adipose tissue and the rat diaphragm methods of bioassay. When undiluted serum was dialyzed against tap water at room temperature an increase in the ILA of the dialyzed serum was observed. The increase, which was localized in a precipitate of denatured globulins, corresponded per ml dialyzed serum to the effect of 100 000 μU crystalline insulin, measured by both methods. It was found that in the presence of calcium ions, a pH value between 7.7 and 8.1 and a temperature of about 37° C favoured the increase in ILA by dialysis. Treatment of serum protein fractions, separated in starch block electrophoresis, with dialysis by a standardized procedure against calcium containing water of low ionic concentration increased the ILA from about 4000 to 450 000 μU ILA per ml of fractionated serum. Of the ILA in the serum protein fractions, before and after dialysis, about two thirds was localized in the alpha globulins, and about one third in the beta-two globulins. A varying proportion of from 50 to 80 per cent of the ILA was suppressed by the addition of anti-insulin serum. Three serum proteins, found in other studies to bind radioactivity when incubated with labelled insulin, showed ILA. Following dialysis, a very marked increase in the ILA of all three globulins was observed. The ILA in the three serum proteins was found to account for most of the ILA in the serum. These findings were obtained by both the adipose tissue and the rat diaphragm bioassay, no significant difference being found between the results obtained by the two assays. Some hypotheses on the state of insulin in the blood are discussed in relation to these observations. It is concluded that the main part of ILA of serum is due to insulin associated with the three globulins, and that the activity of insulin is probably potentiated by binding to serum proteins.