Title

Nature And Function Of Protein Components Of The Milk Of Aquatic Mammals (dolphin, Beta-Lactoglobulins, Alpha-Lactalbumin, Lysozymes)

Date of Award

1986

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Biochemistry and Molecular Biology

Abstract

Milk whey proteins from a single species of three orders, the bottlenose dolphin (Tursiops truncatus) a cetacean, the Florida manatee (Trichechus manatus latirostris) a sirenian and the domestic dog (Canis familiaris) a carnivore have been isolated and characterized.Although the milk of aquatic mammals have been suggested to be devoid of lactose, dolphin milk contained 2% w/v lactose as well as alphalactalbumin (the regulatory protein of the lactose synthase system). The latter protein has been purified and characterized by N-terminal sequence analysis and by its activity in the lactose synthase system. Alpha-lactalbumin and lactose were detected in manatee milk, but in low amounts. Alpha-lactalbumin from dog milk was purified and characterized in the same manner. Beta-lactoglobulins were present in large quantities in the milk of all the species. These proteins were purified and characterized by molecular weight, N-terminal sequence analysis and by their ability to bind retinol. Dolphin milk was found to contain two very divergent beta-lactoglobulins. The complete amino acid sequences of these two proteins (DLG-1 and DLG-2) were determined.Although bovine beta-lactoglobulin has been extensively studied, the function and evolutionary origins of beta-lactoglobulin have not been established. Evidence is presented to show that beta-lactoglobulins are members of a superfamily of low molecular weight proteins which include serum retinol binding protein, alpha(,2u)-globulin, protein HC and alpha(,1)-acid glycoprotein (orosomucoid). The statistical significance of their sequence relationships was established by comparing the numerical scores for comparisons of pairs of sequences with the scores for the alignments of randomized sequences of the same amino acid composition. Sequences of isologous proteins from twelve species were used. Besides being significantly related in sequence, these proteins have similar disulfide bond arrangements and, probably, closely similar three-dimensional structures. The gene structures, which are known for two of the proteins have similarly located non-coding regions (introns) suggesting a common ancestor derivation. Based on similarities in sequence and conformation, it seems that these proteins have similarities in their function at the molecular level. Although the function of only one protein is clearly defined (RBP), evidence suggests that other proteins in this group function as transport proteins for small hydrophobic molecules. All available beta-lactoglobulins are active in binding retinol, and it is suggested that this protein facilitates retinol absorption from the milk in the young animal. (Abstract shortened with permission of author.)

Keywords

Chemistry, Biochemistry

Link to Full Text

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