BAKER, M.E: Albumin, Steroid Hormones, and the Origin of Vertebrates

Albumin, the major serum protein, binds a wide variety of lipophilic compounds including steroids, other lipophilic hormones, phytochemicals and synthetic chemicals that bind to hormone receptors. Albumin has a low affinity for these lipophilic compounds. However, due to albumin’s high concentration in serum, albumin is a major carrier of steroids and lipophilic hormones and a regulator of their access to their receptors. Moreover, albumin functions as a sink for phytochemicals and other lipophilic chemicals from either natural or synthetic sources, which prevents their binding to hormone receptors and other cellular proteins. In this way, albumin protects animals from disruptive endocrine effects that are mediated by these compounds. Sequence analyses indicate that albumin arose in a protochordate, which coincides with the origins of adrenal and sex steroid receptors, the first of which appears to be an ancestral estrogen receptor. We propose that albumin had an important role in the origins of steroid hormone signaling in protochordates and vertebrates about 600 to 530 million years ago, before and during the Cambrian. At that time, animal body sizes and exposure to phytochemicals in food were increasing. Animals in which albumin expression was high had a selective advantage in surviving and reproducing in the presence of toxic phytochemicals and lipophilic chemicals from natural sources.