Evidence for a Genetic Relationship between Avidins and Lysozymes

Abstract

PROTEINS which bind biotin do not seem to be widely distributed in nature, possibly because their function seems to be restricted to that of a somewhat inefficient antibacterial agent. Until recently, avidin had been detected only in the egg-white of birds and in the homologous egg jelly of frogs¹. In 1963 an avidin-like protein was discovered as part of an antibiotic system in culture filtrates of several species of Streptomyces². This protein, called streptavidin, was purified, crystallized and characterized by Chaiet and Wolf³, who found that it resembled egg-white avidin both in possessing four biotin binding sites (one/15,000g) with great affinity for biotin and in some aspects of its amino-acid composition (high tryptophan and threonine, low methionine, proline and histidine). Many of the characteristics of the biotin binding mechanism were also similar. There was a redshift of the tryptophan absorption band accompanying the binding of biotin, and at the same time the biotin protected four tryptophan residues in each sub-unit against oxidation by N-bromosuccinimide (ref. 4 and my unpublished work). Like avidin, streptavidin also bound the dye 4-hydroxy-azobenzene-2′-carboxylic acid giving a new absorption band at 500 mµ which disappeared when the dye was displaced by biotin⁵. Some of the physico-chemical characteristics of these two proteins and of hen egg-white lysozyme are summarized in Table 1.