Analysis and Minimum Molecular Weight of B-lactoglob~jlin

In 1934 Palmer (l), in R. K. Cannan’s laboratory, succeeded in obtaining from the plasma of cow’s milk a crystalline protein with the general properties of a globulin, which was subsequently designated as /3-lactoglobulin (2). According to Palmer (1) and to Sorensen and Sgrensen (3) from 50 to 60 per cent of the whey proteins can bc obtained in the form of this globulin. Detailed physicochemical investigations (solubility (l), electrophoretic mobility (4), ultracentrifuge studies (2, 4), dissociation tendency (5)) indicate the homogeneity of p-lactoglobulin by criteria at present available. Pedersen’s ultracentrifuge investigations on skim milk (2) show that p-lactoglobulin is a protein native to milk and is not an artifact produced during isolation and crystallization. This paper deals with the determination of cysteine, cystine, methionine, tyrosine, tryptophane, arginine, and threonine in /3-lactoglobulin.’ From these data a value for the minimum molecular weight (M,i,.) is obtained which is in close agreement with the molecular weight in solution computed from ultraccntrifuge data (4). The number of amino acids per mole of p-lactoglobulin is ascertained with the aid of additional information’ (amide N and basic amino acid content (6, 7)). The r61e of the hydroxyl groups is discussed. Microor semimicromethods for the accurate determination in proteins of total sulfur (8, 9) and in protein hydrolysates of sulfate sulfur (lo), cysteine (11, la), cystine (11, la), cysteine + cystine (lo), methionine (lo), tyrosine (13), tryptophane (13), and arginine (14) have been described in recent publications from this laboratory. A system of analysis has been developed in which most of these constit,uents are determined by two independent methods. Hydrolysis is carried out in an inert atmosphere with HI in presence of hypophosphite; with HCl, and with HCl in presence of urea (12);2 with n’aOH, and with NaOH-SnClz in sealed tubes containing a minimum of enclosed air (13). In HI digests, we determine methionine as volatile iodide and as homo-