[The primary structure of human free secretory component and the arrangement of disulfide bonds].

The amino-acid sequence and the arrangement of the disulfide bonds of the human free secretory component were completely elucidated by the methods of protein chemistry. The free secretory component is a monomeric glycoprotein (Mr approximately 86000), consisting of 558 amino acids with 7 carbohydrate chains bound to asparagine. The protein contains 20 cysteine residues but, as a special feature, no methionine. The polypeptide chain is divided into five regions of internal homology, 104 to 114 amino acids in length. The 20 cysteine residues form 10 disulfide bonds, 9 of which confirm the internal homology by their characteristic arrangement. The free secretory component also shows homology to immunoglobulins in some sections. A computer-supported tertiary structure is proposed for the free secretory component.