S100ao (alpha alpha) protein is mainly located in the heart and striated muscles.

Concentrations of alpha-S100 protein (S100ao or alpha alpha form, and S100a or alpha beta form) and beta-S100 protein (S100b or beta beta form, and S100a or alpha beta form) in various human tissues were determined by employing the enzyme immunoassay system specific to each subunit of bovine S-100 protein. Immunoreactive alpha-S100 protein was found in the heart and striated muscles at high levels of about or more than 1 microgram/mg soluble protein. Concentrations of beta-S100 protein in those tissues were low (less than 50 ng/mg protein). A considerable content of alpha-S100 protein was also found in the kidney and thyroid gland (about 160 and 100 ng/mg protein, respectively), where the beta-S100 content was less than 5 ng/mg protein. The immunoreactive alpha-S100 proteins in the extracts of heart, kidney and brain were eluted in the same fractions from a column of butyl-Sepharose and in the fractions corresponding to a molecular weight of approx. 20 000 from a column of Sephadex G-100. Both alpha-S100 and beta-S100 proteins were found at a relatively high concentration (100-250 ng/mg protein) in the skin and trachea. The alpha-S100 contents in the other peripheral organs, including gastrointestinal tract, lung, liver, spleen, urinary bladder, gall bladder, uterus, prostate and aorta, were low (less than 50 ng/mg protein). Since brains contain about 300 ng alpha-S100 protein/mg soluble protein, it can be concluded that alpha-S100 (or S-100ao) protein is mainly located in the heart and striated muscle tissues.