Somatomedin levels measured by radioreceptor assay (RRA) or RIA on
acid-
ethanol-extracted or unextracted serum from patients with
uremia are quite variable relative to normal values. We have investigated whether compounds accumulating in uremic serum were interfering with these RRA and RIA measurements.
Insulin-like growth factor I (
IGF-I) and
IGF-II were separated from
carrier proteins by either
Sephadex G-50
acid chromatography or
acid-
ethanol extraction.
IGF-I was measured by RIA, and
IGF-II was determined by rat placental membrane RRA.
IGF-I levels in the
acid-
ethanol extracts of serum from eight uremic adults were only 50% of the levels found in seven normal subjects, and
IGF-II levels in these uremic patients were 350% of normal values. However, these significant differences were not found when comparable serum samples were
acid chromatographed rather than
acid-
ethanol extracted; instead,
IGF-I levels were 343 +/- 168 (mean +/- SD) ng/ml in uremic patients and 325 +/- 54 ng/ml in normal subjects, while
IGF-II levels were 780 +/- 215 ng/ml in uremic patients and 588 +/- 46 ng/ml in normal subjects. To determine whether compounds that interfere with
somatomedin assays were present in the
acid-
ethanol extracts of uremic serum, we
acid chromatographed these extracts and found that a compound eluting in the
carrier protein fraction of the uremic extract artifactually decreased
IGF-I levels measured by RIA and increased
IGF-II levels measured by RRA. We also found that unsaturated
carrier protein binding of
IGF-I and
IGF-II was significantly increased in the
carrier protein fraction of uremic
acid-
ethanol extracts at the dilutions used for
somatomedin assays. We conclude that
acid chromatography of unextracted or
acid-
ethanol-extracted uremic serum is necessary to remove a compound, probably unsaturated
carrier protein, which interferes with RIA of
IGF-I and RRA of
IGF-II. Once this compound is removed, serum
IGF-I levels (RIA) in uremic patients are not different from normal, and serum
IGF-II levels (RRA) are not lower than normal.