Sequestration and degradation of alpha 2u-globulin in rat kidney lysosomes

alpha 2u-Globulin (M.W. = 18,709) is the major protein synthesized in the liver of adult male rats. alpha 2u-Globulin, secreted from the liver, is filtered by the kidneys where about a half of the protein is reabsorbed and the remainder is excreted in the urine. Highly purified lysosomes from rat kidneys were subjected to immunoblot analysis using antiserum against alpha 2u-globulin. A major immuno-reactive band was observed at the position about 1 k dalton smaller than that of the authentic alpha 2u-globulin. The immuno-reactive band in SDS-polyacrylamide gel was excised and the protein, eluted from the gel, was subjected to amino acid sequence analysis by micro Edman method. The N-terminal amino acid sequence of the intralysosomal alpha 2u-globulin was found to be leu-asp-val-ala-lys-leu-asn-gly-... which exactly corresponded to the sequence from the 10th leucine residue of the authentic urinary alpha 2u-globulin. This result suggests that alpha 2u-globulin, sequestered into the kidney lysosomes, is cleaved intralysosomally between the 9th asparagine and the 10th leucine, and the degradative intermediate, thus produced, is accumulated in the lysosomes. The immuno-blot assay gave the value of 210 +/- 70 mg alpha 2u-globulin/g kidney lysosomal proteins. The molecular basis for the resistance of the degradative intermediate of alpha 2u-globulin against further degradation in the kidney lysosomes was discussed in the light of the computer-predicted secondary structure of alpha 2u-globulin.