The estimated molecular weight (MW) of renin is 45K Da. Big renin is hypothesized to be a high MW isoform of renin. Big renin and renin are recovered when renin is purified from rat kidney which is homogenized and combined with pepstatin sepharose, an affinity gel that binds aspartyl proteases such as renin. After elution, two distinct bands are seen in silver stained SDS PAGE gels. These bands are presumed to be renin and big renin based on estimated MW of 45K Da and 60K Da, respectively. Further characterization by immuno-blotting supports this presumption in that antibodies recognize renin and big renin in the pepstatin eluate. The eluate is tested for renin enzymatic activity; the generation of angiotensin I (AI) by incubation with rat angiotensinogen. Before enzymatic activity is tested, some eluate is treated with trypsin which cleaves a peptide segment from prorenin allowing the inactive proenzyme to become enzymatically active. AI is generated when big renin is treated with trypsin, suggesting that big renin may be a form of prorenin or a glycoform of renin. However when big renin is hydrolyzed, there is no measurable carbohydrate content, making it unlikely that big renin is a glycoform of renin. When big renin is separated in an SDS PAGE gel and identification is done using in-gel digest of the protein in the cut band, amino acid sequencing of the peptide fragments identifies big renin as chaperonin. Chaperonin, a heat shock protein, may bind newly synthesized renin. Thus, big renin's association with renin may be due to its protein binding function as chaperonin rather than big renin being an isoform of renin. Table

Table 1 Summary of physicochemical characteristics of renin and big renin.
Full size table