Dipeptidyl peptidase IV (DPP IV) in normal human serum was purified 14,400-fold with a 25% yield to homogeneity. The molecular weight of the purified enzyme was approximately 110,000 on SDS-PAGE, almost the same as that of human kidney membrane-bound DPP IV. No difference was found between the two enzymes enzymologically and immunologically, either in substrate specificity, susceptibility to inhibitors, or cross-reactivity with an anti-rat kidney DPP IV antibody, or in their ability to bind adenosine deaminase. However, the N-terminal amino acid sequence of serum DPP IV lacked the transmembrane domain of the membrane-bound enzyme and started at the 39th position, serine, from the N-terminus predicted from the cDNA nucleotide sequence. These results suggest that membrane-bound DPP IV loses its transmembrane domain upon release into the serum, and that its structure on the plasma membrane is not required for its binding to adenosine deaminase.
Adenosine deaminase (ADA) plays an important role in the immune system, and its activity is composed of two kinetically distinct isozymes, ADA1 and ADA2. ADA2 is a major component of human plasma total ADA activity and ADA2 activity is significantly elevated in patients with various diseases such as HIV infection and chronic hepatitis. However, relatively little is known about ADA2 because of difficulties in purifying this enzyme. In this study we succeeded in purifying human plasma ADA2 and demonstrate the extracellular secretion of ADA2 from human peripheral blood monocytes stimulated with phorbol 12-myristate 13-acetate and calcium ionophore.
Infections in humans by Bartonella bacilliformis, but not Bartonella henselae, are characterized by invasion of red cells. Supernatants of culture medium from B. bacilliformis and B. henselae each contain a protein which causes invagination of membranes of human red cells and formation of intracellular vacuoles. These two proteins are very similar in molecular mass, heat stability and mechanism of action. B. henselae does not bind to human red cells, but human red cell ghost membrane proteins were recognized by both bacteria, five by B. bacilliformis and the same five, and one additional protein by B. henselae. Two of these proteins had molecular masses consistent with actin and spectrin. Actin binds to five electroblotted outer membrane proteins from B. henselae and four of these proteins are retained on an actin-Sepharose column.
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