Albumin's role in steroid hormone action and the origins of vertebrates: is albumin an essential protein?

Albumin, the major serum protein, binds a wide variety of lipophilic compounds including steroids, other lipophilic hormones and phytochemicals 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 regulator of their access to their receptors. Moreover, albumin functions as a sink for phytochemicals, which prevents their binding to hormone receptors and other cellular proteins, protecting animals from disruptive phytochemical-mediated endocrine effects. We propose that these properties of albumin were important in protochordates and vertebrates about 550 to 520 million years ago, just before and during the Cambrian. At that time, animal body sizes and exposure to phytochemicals in food were increasing, and animals in which albumin expression was high had a selective advantage in surviving and reproducing in the presence of toxic phytochemicals. This hypothesis that albumin has essential function(s) in mammalian endocrine physiology can be tested by comparing the effects of phytochemicals in Nagase rats that have 1/1000 the normal albumin concentration or in mice in which the albumin gene is knocked out with those in normal rats and mice.