XYLOSYL TRANSFER TO AN ENDOGENOUS RENAL ACCEPTOR - CHARACTERISTICS OFTHE REACTION AND PROPERTIES OF THE PRODUCT

In the course of a study of UDP-xylose:proteoglycan core protein xylos yltransferase (EC 2.4.2.26), another xylosyltransferase was discovered in the soluble fraction of a rat kidney homogenate. The latter enzyme catalyzed [H-3]xylosyl transfer from UDP-[H-3]xylose to an endogenous acceptor and yielded a product in which the xylose was bound by an al kali-stable linkage. It was therefore concluded that the acceptor was not the core protein of one of the proteoglycans containing a xylose - -> serine linkage, since this linkage is cleaved by alkali. The [H-3]x ylose-labeled product emerged with the void volume when chromatographe d on Sephadex G-50, it was precipitated by trichloroacetic acid, and i t had a mobility on sodium dodecyl sulfate-polyacrylamide gel electrop horesis corresponding to a molecular mass of about 32,000 Da. Digestio n with trypsin or alpha-amylase degraded the labeled product to small fragments, as determined by gel chromatography, suggesting that it was a glycoprotein related to glycogen. A product of similar characterist ics was formed when UDP-[H-3]glucose was substituted for UDP-[H-3]xylo se as the glycosyl donor, and the two nucleotide sugars were mutually competitive in the respective transfer reactions, indicating that they were substrates for the same enzyme. On the basis of these findings, it was tentatively concluded that the xylosyltransferase and its accep tor were the renal form of glycogenin.