2-DIMENSIONAL ELECTROPHORETIC ANALYSIS OF PROTEINS OF BOVINE MAMMARY-GLAND SECRETIONS COLLECTED DURING THE DRY PERIOD

Proteins in mammary gland secretions, collected from Holstein cows dur ing the dry period, were analyzed by preparative isoelectric focusing, followed by SDS-PAGE. Protein profiles changed throughout the dry per iod. intact casein bands were present throughout the dry period, but i n reduced proportions from d 7 after dry-off through d 7 prepartum. Br eakdown fragments of casein were particularly apparent in secretions f rom d 7 to 21 of the dry. period. A fragment of beta-CN (approximately 13 kDa) was identified in secretions collected on d-1 prior to dry-of f through d 21 of the dry period. Beta-Lactoglobulin generally focused in a limited number of fractions, but, in the sample at d 21 prepartu m, beta-LG focused over a broad pH range. Lactoferrin was found in all fractions after isoelectric focusing, but specific degradation produc ts of lactoferrin were apparent only at certain times during the dry p eriod. This method is a valuable approach for separation of proteins f rom the complex mixtures found in mammary secretions during the dry pe riod. A number of peptides identified by this method may be generated from breakdown of milk proteins in the mammary gland during the dry pe riod and could have a role in the involuting gland.