On the cysteine and cystine content of proteins. Differences between intracellular and extracellular proteins.

Analysis of published data on the cysteine and half-cystine content of proteins indicates that most intracellular proteins may be classified as sulfhydryl proteins (those containing cysteine but little or no half-cystine) and that such sulfhydryl proteins have a low cysteine content. The mean systeine content found for 32 intracellular mammalian proteins was 1.6% and intracellular proteins of many bacteria have similar or lower values. Extracellular mammalian proteins are primarily disulfide proteins (those containing half-cystine but little or no cysteine) have a high half-cystine content, the mean value found for some 34 extracellular mammalian proteins being 4.1%. This is contrasted with many of the extracellular proteins from facultative bacteria which are cyst(e)ine-free proteins, being lacking in both cysteine and half-cystine. These and related observations are interpreted in terms of the evolution of life in a reducing atmosphere and the subsequent transition to an oxidizing environment. It is suggested that disulfide proteins evolved primarily after the accumulation of oxygen in the atmosphere.