Reaction catalyzed by galactosylgalactosylxylosylprotein 3-beta-glucuronosyltransferase (2.4.1.135)

		+			=			+
	UDP-glucuronate	+		3-beta-D-galactosyl-4-beta-D-galactosyl-O-beta-D-xylosylprotein	=		UDP	+		3-beta-D-glucuronosyl-3-beta-D-galactosyl-4-beta-D-galactosyl-O-beta-D-xylosylprotein