ABSTRACT. In this study, bovine β 2 -m was purified from urine by ion-exchange chromatography and gel chromatography, and the characteristics were compared with those of colostral β 2 -m by the immunological reactivity, isoelectric points, peptide map, and amino acid sequence. The characteristics of purified urinary β 2 -m were consistent with those of the colostral β 2 -m. The urinary and colostral β 2 -m possessed the same polypeptide chain consisting of 98 amino acids, and its molecular weight is 11.8 kDa. Furthermore, four isoforms of β 2 -m were found. The isoelectric points were different from each other. KEY WORDS: amino acid sequence, bovine β 2 -microglobulin, isoform, peptide map, purification.J. Vet. Med. Sci. 62(8): 867-874, 2000 β 2 -microglobulin (β 2 -m) is a single polypeptide chain of 100 amino acids (molecular weight = 11.800), which is found in body fluids and on the surfaces of all nucleated cells [7]. It is also the light chain of the class I major histocompatibility antigens. Based on amino acid sequences, β 2 -m is homologous to the α 3 -domains of the heavy chain for the class I histocompatibility antigens and to the immunoglobulin constant domains [21,23].Human β 2 -m is used for evaluation of kidney function, because β 2 -m is eliminated by renal glomerular filtration and subsequently reabsorbed almost completely into the proximal tubule cells, where it is metabolised [12]. Elevated serum levels of human β 2 -m reflect glomerular dysfunction, and its increased concentration in urine reflects proximal tubule disorder [31]. Recently, it has been found that the concentration of serum β 2 -m increased in association with various tumours [20]. Immunological studies have reported that β 2 -m may be involved in antigenic recognition of T-lymphocytes [4,26]. In hemodialysis, it was found that β 2 -m plays the role of an amyloid forming protein, and is one of the causes of kidney amyloidosis in long time hemodialysis treatment. In other species, a relationship between β 2 -m and kidney function, and between its protein and the immunological system including various tumors, have been reported, but few clinical studies are reported. β 2 -m was first isolated from urine of patients with chronic cadmium poisoning [2]. It was later isolated from the urine of various animals (e.g. rat, rabbit, and guinea pig) with renal dysfunction [3,5,17]. In ruminant (bovine and goat) β 2 -m was isolated from milk [8,10]. The isolation of bovine β 2 -m was carried out first by Gloves and Greenberg from colostrum [8]. In a previous study, we demonstrated that a low molecular protein increased in the urine of bovine with renal dysfunction, however, we did not identify this protein [15]. Thus, in this study, we attempted to isolate bovine β 2 -m from urine, and compare it with the biochemical characteristics of β 2 -m from colostrum in order to confirm that this protein is not a metabolite of β 2 -m.
MATERIALS AND METHODS
Animals:Five healthy female Holstein cows were used in this experiment. Three of them were use...