Insulin-like activity by fat pad assay and immunoreactive insulin. II. Their occurrence in fractions by gel filtration of serum

In a previous report, serum insulin was extracted with efficient recovery by salt-ethanol extraction.

As the second method of separation of serum insulin gel filtration was adopted. Dog sera obtained from the pancreatic and the femoral veins were separated into two fractions (A and B) with Sephadex G-50. Fractions A and B corresponded respectively to the major peak of serum proteins and the portion where I-131-insulin added to serum was eluted. Insulin-like activity (ILA) was assayed by rat fat pad and immunoreactive insulin (IRI) by a double antibody method. Significant ILA was demonstrated in both fractions. IRI in serum appeared mostly in fraction B. ILA/IRI ratio was closer to 1.0 in fraction B than in serum. Fraction A tended to have fairly uniform ILA irrespectively of origin of serum, often exceeding the ILA of serum. Fraction A from a depancreatized dog serum had similar degree of ILA. Serum and both fractions stimulated various phases of fat pad metabolism similarly to insulin, including glycogen synthesis. Assays based on four different metabolic parameters gave essentially the same ILA values for both serum and fractions. ILA of fraction A differed from insulin in its nonneutralizability with anti-insulin serum, its poor suppressibility by cysteine, and the lack of parallelism of its regression line with that of insulin. Simple dilution and reconcentration of serum resulted in an increase in its ILA. Presumably, fraction B is primarily due to serum insulin with similar immunological and biological activities to pancreatic insulin. Whether or not ILA of fraction A is related to insulin remains unknown.

Insulin-like activity by fat pad assay and immunoreactive insulin. I. Their correlation in native serum acid-ethanol extract and salt-ethanol extract

In view of controversies on the relationship of ILA (insulin-like activity) and IRI (immunoreactive insulin) in blood, removal of extra factors other than insulin was attempted by means of salt-ethanol (acid-ethanol-NaCl) or acid-ethanol extraction methods prior to assay. The saltethanol procedure, a modification of Romans' method for the extraction of pancreatic insulin, proved to be more efficient in extraction. ILA was assayed by rat fat pad method using C-14-O2 production from glucose-U-C-14 as index. IRI was assayed by a double antibody radioimmunoassay. Dog sera obtained from the pancreatic and femoral veins were chiefly used. Recoveries of endogenous IRI by saltethanol and acid-ethanol methods were 90 per cent and 72 per cent respectively, which were almost identical with recoveries of I-131-insulin added to sera. The ILA of the extracts differed considerably from the ILA of the original sera; the recovery scattered widely beween 5 and 116 per cent. In contrast to wide discrepancies between ILA and IRI in the native sera, ILA and IRI in the extracts were much closer to each other. There was a good parallelism between regression lines of ILA in the extracts and of standard insulin, but deviation was significant between the regression lines of insulin and untreated sera. The ILA in the extracts of dog and human serum was mostly suppressed with antiinsulin serum. Thus, endogenous IRI behaves like pancreatic insulin during extraction procedure and appears to possess biological activity corresponding to its immunoreactivity.