Enzymic and immunological activity of various B proteins of lactose synthetase

Amino terminal sequence, processing, and biological activity of porcine pre-alpha-lactalbumin

Polyadenylated RNAs isolated from bound polysomes of a lactating sow's mammary gland, were translated in a cell-free system and in vitro synthesized alpha-lactalbumin was immunoprecipitated and radiosequenced. The translation product was found to contain an amino terminal extension of 19 amino acid residues, very similar to its ovine counterpart, that was selectively removed when translation was carried out in the presence of rabbit mammary microsomal membranes. Assays of porcine pre-alpha-lactalbumin for activity on galactosyltransferase showed that the preprotein can also interact with and modify the specificity of the enzyme, as indicated by de novo synthesis of lactose.

Antibodies to lactalbumin interfere with its radioimmunoassay in human plasma

Two radioimmunoassays for human lactalbumin have been established using a rabbit antiserum. One assay uses a second antibody to separate bound from free label; the other uses polyethylene glycol to precipitate gamma globulin non-specifically. We have confirmed that about half the normal human population have a substance in their blood which inhibits the binding of lactalbumin to the rabbit antibody. Comparison of the two assays has demonstrated that this material is not lactalbumin but a naturally occurring antibody. We have shown that it is in the IgG fraction of human plasma and is probably a cross-reacting antibody to bovine lactalbumin. None out of fifteen males and fourteen out of fifty eight non-pregnant, non-lactating females had low levels of lactalbumin in the their blood (0.6--2.0 ng/ml). Our assay could not detect a statistically significant difference between normal women and those with either benign breast disease or metastatic mammary carcinoma.

The interference of endogenous antibodies to bovine lactalbumin in the radioimmunoassay of human lactalbumin in serum

Using a radioimmunoassay for the milk protein lactalbumin in human serum, we have found that falsely high results can be caused by cross-reacting human antibodies to bovine lactalbumin. A reliable method of circumventing this problem is described. When such interference had been eliminated, circulating lactalbumin was not detected in normal men or post-menopausal women, but could still be found in a proportion of non-pregnant, non-lactating pre-menopausal women.

Isolation and characterization of casein mRNAs from lactating ewe mammary glands

RNA from bound polysomers of lactating ewe's mammary gland directs the synthesis of the three major milk proteins (alphas, beta and kappa-caseins) in a cell-free system derived from rabbit reticuyocytes. The "in vitro" product was identified by immunoprecipitation with specific antibodies and by electrophoresis in SDS polyacrylamide gel. Each of these messengers was purified from 20 to 25 fold from total membrane-bound polysomal RNA using poly U-Sepharose chromatography. This purified fraction assayed in a reticulocyte cell-free system is able to direct also the synthesis of 2 minor secretory proteins (beta-lactoglobulin and alpha-lactalbumin). The messenger RNAs purified by hybridization to poly U-Sepharose have a sedimentation coefficient of about 12 S and an apparent molecular weight of approximatively 3.5 s 10-5 daltons was observed by polyacrylamide gel electrophoresis under denaturing contitions. This value which correspond to about 900 nucleotides is significantly greater than the number expected for coding milk proteins.

Immunological cross-reactions of alpha-lactalbumins from different species

Four rabbit antibodies have been prepared, which are specifically directed against alpha-lactalbumins from different sources; namely human, cow, goat and sheep milk. Each of these antibodies was tested for its ability to react with, separately, each of the four proteins. The immunological reactions were assessed by means of different techniques: double immunodiffusion in agar gel as well as affinity chromatography of antibodies, using antigens covalently bound to an insoluble matrix. In each case, the strongest reaction was observed between homologous antibody and (matrix-bound) antigen; heterologous antigens were, however, also capable of cross-reaction. Whereas no cross-reaction between human alpha-lactalbumin and antibodies against the bovine protein could be evidenced by immunodiffusion, the occurrence of soluble complexes has been disclosed by means of a gel filtration technique.