Identification of a Protein Increasing in Serum of Nagase Analbuminemic Rats Bearing Intestinal Tumors as an Isotype of T‐Kininogen

Increase in an unidentified protein was observed in serum of Nagase analbuminemic rats (NAR) bearing intestinal tumors induced by azoxymethane. This protein seemed to be a polymer of a protein of 73 kDa as estimated by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis, and so was tentatively named 73K‐protein. The serum concentration of 73K‐protein in NAR bearing intestinal tumors was 11.9 ± 2.2 mg/ml (mean ± SD, n = 5), whereas that in control NAR was 2.0 ± 0.2 mg/ml. Increase of the serum 73K‐protein level was also observed in Sprague‐Dawley rats bearing intestinal tumors, skin tumors, subcutaneous sarcomas, or mammary tumors and in ACI rats bearing urinary bladder tumors. On double immunodiffusion analysis, the 73K‐protein was not detected in mouse, guinea pig, pig, horse, or human serum. A cDNA clone bearing the sequence encoding 73K‐protein was isolated from a cDNA library constructed from rat liver mRNA. The nucleotide sequence of the 73K‐protein showed 98.8% and 96.9% homologies with the sequences of the 3′‐proximal domains of the cDNAs for TI‐ and TII‐kininogen, respectively. Therefore, the 73K‐protein was concluded to be an isotype of T‐kininogen.