Pig β-Lactoglobulin I(Sus scrofa domestica,Artiodactyla). The Primary Structure of the Major Component

Abstract

.beta.-Lactoglobulins from pooled milk (Sus scrofa domestica) are isolated and characterized. The complete primary structure of the major .beta.-lactoglobulin component I is presented. The amino-acid sequence was elucidated by automated Edman degradation of tryptic peptides and cyanogen bromide cleavage products in a liquid phase sequencer. The tryptic and cyanogen bromide peptides were separated by reverse-phase (RP-2) or size exclusion (TSK 2000 SW) high performance liquid chromatography. Pig .beta.-lactoglobulin is composed of only 159 amino acids in contrast to other .beta.-lactoglobulins which contain 162 or 166 amino acids. Sequence alignment with previously sequenced .beta.-lactoglobulins was obtained by introducing two gaps at positions 115 and 151-152. Thus bovine .beta.-lactoglobulin A reveals 62 amino-acid substitutions. The phylogenetic distance from horse .beta.-lactoglobulin I and II is indicated by 49.4% and 62% amino-acid exchanges, respectively. Pig .beta.-lactoglobulin is a mixture of two chains with Gln or Thr at position 119. The free thiol group is localized at position 59. The structural and functional aspects of .beta.-lactoglobulins and its role in vitamin A (retinol) transport are discussed.