Amino acid sulfur as a source of sulfate for sulfated proteoglycans produced by Swiss mouse 3T3 cells

The uptake of sulfate by Swiss mouse 3T3 cells is blocked in the presence of 1 mM 4-isothiocyano-4'-acetamido-stilbene-2,2-disulfonic acid (SITS). In the absence of an exogenous source of sulfate, glycosaminoglycans produced by cells in the presence of the inhibitor are sulfated to the same extent as those produced by cells grown in its absence. The sulfate utilized in the absence of medium sulfate has been identified as that produced by the oxidation of the sulfur present in the amino acids cysteine and methionine. This finding indicates that, under conditions of restricted exogenous sulfate, caution is needed in the interpretation of data obtained with the use of 35S]methionine and/or 35S]cysteine as a general protein label, since both tyrosine and a variety of types of protein-linked carbohydrate chains may be modified by sulfation.