Isolation and amino acid composition of the isotypes of a rat Clara cell specific protein

A protein of molecular weight about 10,000 (Clara cell protein C) present in lung lavage fluid and specific to Clara cells has been shown to have three isotypes. The isotypes have been individually isolated and purified by a combination of molecular sieving, ion exchange chromatography, column chromatofocusing, and reverse phase chromatography. The protein was monitored by immunoblotting and its purity tested by silver staining of the gel following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. About 33.7% of the 10-kDa protein present in starting rat lavage was recovered in final pure preparation. The 10-kDa Clara cell protein amounted to 0.38% of the total protein in rat lung lavage fluid. The three isotypes were present in a ratio of 1.0:2.8:8.8, the amounts increasing with decreasing pI. None of the isotypes bound to concanavalin A. The amino acid compositions of the three isotypes were similar and were remarkable in that the contents of leucine, glutamic acid, aspartic acid, serine, and proline were high, tyrosine was present in low amounts, and methionine and histidine were absent.