Characterization of solubilized GlcAT-1 (UDP-GlcA: nLcOse4Cer beta 1-3 glucuronyltransferase) activity from embryonic chicken brain and its inhibition by D-erythro-sphingosine.
Glycolipid glucuronyltransferase activity (GlcAT-1) has been solubilized and characterized from 19-day-old embryonic chicken brain Golgi-rich membranes. The enzyme catalyzes the biosynthesis in vitro of GlcA beta 1-3nLcOse4Cer glycolipid using neolactetraosylceramide (nLcOse4Cer, Gal beta 1-4GlcNAc beta 1-3Gal beta-1-4Glc-Cer) as the substrate. The membrane-bound enzyme shows optimum activity in the presence of neutral detergents such as Triton CF-54, Triton DF-12, and Nonidet P-40. Approximately 60% of the enzyme activity can be solubilized from the Golgi membrane by Nonidet P-40. The solubilized GlcAT-1 activity is inhibited by different salts such as NaCl, NaBr, NaI, and NaOAc, but not by sodium fluoride (up to 0.4 M concentration). Desialyzed alpha 1 acid glycoprotein (SA alpha 1AGP) can be used as a substrate for glucuronyltransferase. Competition studies between glycolipid (nLcOse4Cer) and glycoprotein SA alpha 1AGP) substrates show a mixed type of inhibition. Phospholipids, in particular phosphatidylglycerol, stimulate solubilized GlcAT-1 activity, while D-erythro-sphingosine, a metabolite of glycosphingolipids, is inhibitory (50% inhibition at 0.8 mM D-erythro-sph). These results demonstrate that both phospholipid as well as sphingosine might be involved in modulating glucuronyltransferase activity.