Process of the combination of alpha-lactalbumin with lysosomes in mammary epithelial cells

Creation and phenotypic analysis of alpha-lactalbumin-deficient mice

alpha-Lactalbumin is an abundant milk-specific calcium metalloprotein which has an evolutionary relationship to lysozyme. It modifies the substrate specificity of a Golgi galactosyltransferase by forming the lactose synthetase binary complex. Lactose, together with other sugars and diffusible ions, is responsible for the osmotic pressure of milk. To assess the involvement of alpha-lactalbumin in lactogenesis, alpha-lactalbumin-deficient mice were created by disrupting the gene by homologous recombination in embryonic stem cells. Homozygous mutant mice are viable and fertile but females cannot feed their offspring. They produce a highly viscous milk that pups appear to be unable to remove from the mammary gland. This milk is rich in fat and protein and is devoid of alpha-lactalbumin and lactose. The phenotype of heterozygous mice was found to be intermediate, with a 40% decrease in alpha-lactalbumin but only a 10-20% decrease in the lactose content of their milk compared with wild-type animals. These results emphasize the key function of alpha-lactalbumin in lactogenesis and open new opportunities to manipulate milk composition.

Complete nucleotide sequence of bovine alpha-lactalbumin gene: comparison with its rat counterpart

The nucleotide sequence of the bovine alpha-lactalbumin gene, whose organization is very similar to that of its rat counterpart, was deduced from the analysis of 2 lambda clones isolated from a HindIII genomic bank. The 3090 sequenced nucleotides comprise 738 bp upstream from the transcription unit (approximately 2 kb) which contains 4 exons of 160, 159, 76 and 330 bp separated by 3 introns of 321, 473 and 504 bp. Comparison with the rat alpha-lactalbumin gene shows similar percentages of homology between the 4 cognate exons. Since only the first three exons are homologous to the corresponding exons of the lysozyme gene, it is suggested that the 4th exons of alpha-lactalbumin and lysozyme genes have different origins. The bovine alpha-lactalbumin mRNA is 725 nucleotides long, excluding the poly(A) tail. The reading frame and the flanking 5' and 3' untranslated regions contain 429, 27 and 269 nucleotides, respectively. The derived amino acid sequence differs at 10 positions from that determined directly on mature alpha-lactalbumin.

Alpha-lactalbumin in bovine serum: relationships with udder development and function

Concentrations of alpha-lactalbumin in blood from cattle in various physiological states were measured as an index of udder development and function. Included were primiparous heifers during gestation and the peripartum period, nonlactating, nonpregnant cows hormonally induced into lactation, and cows milked two or three times daily in early, middle, or late lactation. Concentrations of alpha-lactalbumin in serum increased in two phases during gestation. Initial values (7.3 ng/ml, up to 120 d prepartum) rose, then leveled at 29.9 ng/ml on d 120 to 30 prepartum. Concentrations subsequently increased, averaging 133.2 ng/ml over the 30 d prior to parturition. During the peripartum period, alpha-lactalbumin rose from 221.2 ng/ml on d 4 prepartum, peaked at calving (918.8 ng/ml), then declined, stabilizing at approximately 500 ng/ml (1.5 to 3 d postpartum). Concentrations of alpha-lactalbumin in cows induced to lactate were low on d 1 to 9 of hormone treatment (15.7 ng/ml), rose to a maximum on d 17 (803.4 ng/ml), then fell to a plateau (185.8 ng/ml) on d 21 to 25. alpha-Lactalbumin concentrations were higher in early (101.9 ng/ml) than in middle or late lactation (81.4 and 79.2 ng/ml, respectively). Concentrations were also greater in twice versus thrice milked cows (101.9 vs. 73.0 ng/ml). Changes in alpha-lactalbumin concentrations in serum are associated with developmental and functional status of the udder. The measurement provides a noninvasive method to assess mammary gland activity.