cDNA sequence of lactophorin and and its emulsification function

乳霍蛋白(Lactophorin，LP)是牛乳蛋白质中的一种主要糖蛋白，由27 kDa和17 kDa两种主要糖肽组成。克隆了Lp(27kLP)27 kDa糖肽的编码基因。该蛋白由621个碱基组成，编码18个氨基酸的信号肽和135个氨基酸的成熟蛋白，推导出的成熟氨基酸序列与Sorensen和Petersen最近化学测定的蛋白质-蛋白内酯组分3(PPC3)的序列相同。与凝胶过滤分离的蛋白质-蛋白内酯组分-3(PPC3)相比，LP具有最高的乳化活性、乳化稳定性、比表面积和最小的球体中值直径。由1%乳脂和25%乳脂制备的乳状液具有2500的乳化活性，稳定性好，乳胶中值直径为2.45微米，比表面积为31.7×10~(-3)cm~(-2)/cm~3。乳状液中低密度脂蛋白含量增加到60 mg/g乳脂时，乳状液的乳化活性、乳状液稳定性和比表面积增加，中值直径减小。从疏水氨基酸残基在一级结构和二级结构的α螺旋轮中的分布来探讨LP的乳化能力。

Lactophorin (LP) is a major glycoprotein in the proteose-peptone fraction of bovine milk and constituted of two major glycopeptides (27 and 17 kDa). A cDNA encoding 27 kDa glycopeptide of LP (27kLP) was cloned. It was revealed to consist of 621 bp, which encoded 18 amino acids for signal peptide and 135 amino acids for mature LP.The deduced mature amino acid sequence is identical to that of proteose-peptone component 3 (PPC3), which has recently been chemically determined by Sorensen and Petersen [J.Dairy Res.60,535, (1993)].LP had the highest emulsifying activity, emulsion stability, and specific surface areas and smallest median diameter of globules in comparison with those of other fractions of proteose-peptone component-3 (PPC3) separated by gel filtration. Emulsions prepared from 1% LP and 25% milk fat had 2500 of emulsifying activity and was highly stable and median diameter of their glubules was 2.45mum and their specific surface area was 31.7x10^3cm^2/cm^3. In addition, increasing of LP content up to 60 mg/g of milk fat in emulsion increased emulsifying activity, emulsion stability, and specific surface areas and decreased median diameter. Emulsifying ability of LP is discussed from the distribution of hydrophobic amino acid residues in the primary structure and in alpha-helical wheel of the secondary structure of LP.