Amino acid sequence homology between human placental protein 14 and beta-lactoglobulins from various species.

The primary structure of 22 N-terminal amino acid residues of placental protein 14 was determined by automated Edman degradation with a gas-phase sequencer. This protein, isolated from the human placenta and its membranes, was considered pure as evidenced by a single N-terminal amino acid sequence M D I P Q T K Q D L E L P K L A G T W H S M. It shows significant sequence homology with horse, bovine, buffalo, sheep and goat beta-lactoglobulins. We found 13 identities out of 22 possible matches with horse beta-lactoglobulin. beta-lactoglobulins from several animal species have been found to bind retinol. Among the identical residues there is one tryptophan at position 19 which is conserved in beta-lactoglobulins and is also found in the human retinol-binding protein at the corresponding position. These data suggest a common origin of PP14 and beta-lactoglobulins.