Apolipoprotein D is the major protein component in cyst fluid from women with human breast gross cystic disease.

GCDFP(gross-cystic-disease-fluid protein)-24, a progesterone-binding protein present in large amounts in cyst fluid from human breast gross cystic disease, was purified in a one-step procedure by size-exclusion h.p.l.c. Peptide fragments obtained by trypsin digestion of the intact protein were purified by reverse-phase h.p.l.c. and analysed for their amino acid composition and subjected to automated Edman degradation. A search of the National Biomedical Research Foundation Data Bank revealed that all the sequenced tryptic peptides from protein GCDFP-24 matched perfectly with regions present in the amino acid sequence determined for human apolipoprotein D. Additional data on N-terminal sequence of the unblocked proteins, carbohydrate-attachment sites, amino acid composition and molecular-mass estimations supported the identity between both molecules. On the basis of this identity a possible role of apolipoprotein D in progesterone transport is proposed.