We demonstrated previously (
Cancer Res., 42: 4964-4969, 1982) that a
tumor-associated factor was consistently present in the plasma of over 100 human
cancer patients with
tumors at 31 different sites. The plasma of healthy controls had very low activity in the biochemical assay. In the present study, we show by a combination of molecular sieving and assay of
nuclear RNA transport that the
tumor-associated factor, which has a molecular weight of 60,000, is undetectable in the plasma of healthy adults. The low activity reported earlier is due to three normal cell factors of markedly different molecular weight. Furthermore, the
tumor factor is shown to be absent from the plasma of male and female patients hospitalized for a variety of nonmalignant surgical conditions. Only the plasma from patients who were pregnant, suffered from
chronic renal failure, or had recent
myocardial infarction gave false positives in the biochemical assay. However, in these cases, the activity was due to an increase in the normal tissue-associated factors and not to the appearance of the Mr 60,000
tumor-associated factor. The factor is present in amniotic fluid, confirming that it is a fetal factor which does not cross the placental barrier. Thus, it may be classified as an oncofetal factor. All four factors found in the plasma were identified in the cytosol from a human
tumor. In summary, the
tumor-associated factor appears to be
tumor specific and can be unambiguously identified by bioassay of the plasma factors eluting from
Sepharose CL-6 B columns in the Mr 60,000 region. It can also be identified by examination of
sodium dodecyl sulfate:
polyacrylamide gel electrophoretograms of the appropriate
Sepharose CL-6 B fractions after removal of
albumin.