EFFECT OF LIPIDS ON GLYCOPROTEIN SULFOTRANSFERASE ACTIVITY IN RAT SUBMANDIBULAR SALIVARY-GLANDS

Although glycoprotein sulphation has been implicated in the processing of salivary mucin, little is known about the regulation of the enzyme responsible for this event. Using desulphated glycoprotein as sulphat e acceptor, the glycoprotein sulphotransferase (GPST) from Golgi membr anes of submandibular salivary gland was used to study the effect of v arious lipids on its activity. The GPST activity in the Golgi membrane was 0.7 pmol/mg protein per min and the activity was extractable by T riton S-100. The K-m of the solubilized GPST for glycoprotein and 3'-p hosphoadenosine 5'-phosphosulphate (PAPS) were 11 and 0.2 mu M, respec tively. Among the various lipids tested, phosphatidylinositol and sphi ngosine stimulated the GPST activity, while other lipids such as sphin gomyelin, phosphatidylcholine and phosphatidylserine did not produce a significant effect. At 12 mol% (when expressed as mol% of sphingosine to total phospholipids plus Triton X-100) of sphingosine concentratio n, the enzyme activity was increased nearly 1.7-fold. The stimulatory effect of sphingosine was accompanied by a significant decrease in K-m for glycoprotein from 11 to 2 mu M but the increase in V-max was smal l. In contrast, the sphingosine effect did not change the K-m for PAPS but increased the V-max nearly two fold. Of the two sphingosine analo gues tested, threosphinganine and erythrosphinganine had a lesser stim ulatory effect than sphingosine. Stearylamine was partially active, wh ereas the amino acids (glutamate, aspartate, glutamine, asparagine and serine) were not. These observations and our earlier finding of tryos ylprotein sulphotransferase inhibition by sphingosine demonstrate dive rse sphingosine effects on the posttranslational sulphation involved i n the processing of salivary proteins and suggest an important role fo r sphingosine in the regulation of salivary protein sulphation.