Affinity Labeling of Hormone-Specific Proteins*

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Abstract
AFFINITY labeling for studying the amino acid topography of specific binding sites in biologically important proteins has emerged as an important biochemical technique during the past three decades. Hormone-specific proteins have been the subjects of affinity labeling during the past two decades, beginning with the analysis of the amino acids which constitute the catalytically active site of the enzymes involved in steroid biosynthesis. More recently, hormone-specific receptor and transport proteins have been characterized, and the amino acid compositions of their binding sites have been described by affinity labeling. The reagents used for affinity labeling are analogs of hormones which produce active site-directed irreversible inhibition in the proteins which they attack. Therefore, they have been regarded as potential new drugs for controlling fertility, treating endocrine disorders, and for treating hormone-sensitive cancers. The three major categories of affinity labeling hormone analogs have been classified according to the reagent groups they possess and the mechanisms by which they inhibit the proteins. Affinity alkylating analogs possess a reagent group which at all times is reactive toward certain amino acids. Photoaffinity labeling ana- logs are only reactive while they absorb light energy, and in the energized state they can react with most amino acids. Autoinactivating substrates have thus far only been used with enzymes because they are reactive toward amino acids only after an enzyme has converted the substrates to a reactive form. All three categories of affinity labeling hormone analogs cause irreversible inhibition when they react with an amino acid at a binding site of a hormone-specific protein.

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