JBC, Vol. 251, Issue 7, 1969-1974, Apr, 1976
Bovine secretory component. Isolation, molecular size and shape, composition, and NH2-terminal amino acid sequence
R. S. Labib, N. J. Calvanico and T. B. Tomasi
Bovine free secretory component was purified from whey by salt
precipitation, gel filtration, DEAE-cellulose and phosphocellulose
chromatography, and immunoadsorption. It was obtained in immunologically
pure form and in 56% yield. The Stokes radius of pure free secretory
component was found to be 4.3 nm by gel filtration, and an (see article) of
4.1 S was determined by the ultracentrifuge. The molecular weight was
79,000 by sodium dodecyl sulfate gel electrophoresis and by sedimentation
dquilibrium in the ultracentrifuge, using a v of 0.73 determined by
ultracentrifugation in D2O and H2O. A minimal axial ratio of approximately
5 was calculated. Amino acid analysis of bovine free secretory component
showed remarkable similarity to that of human, dog, and rabbit but
carbohydrate analysis showed significant differences. In contrast to the
human, bovine free secretory compoennt has 2 methionine residues/mol. The
NH2-terminal sequence was found to be
Lys-Ser-Pro-Ile-PPHE-Gly-Pro-Glu-Glu-Val-Asp-Ser-Val. This sequence is
identical with that the human and dog. However, the poor immunological
cross-reactivity between the dog, human, and bovine proteins suggests that
significant structural differences will be found in other regions of the
molecule.
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