Composition and biogenesis of complex carbohydrates of ox adrenal chromaffin granules

Abstract

The complex carbohydrate composition of ox adrenal chromaffin granules has been determined, since such analytical data are a prerequisite for studies on the biogenesis of these carbohydrates and for an understanding of their functional role in the organelle. The predominant sugars in the glycoproteins of the membrane are mannose, galactose and N-acetylglucosamine, whereas galactose, N-acetylgalactosamine and sialic acid are present in high concentration in the glycoproteins of the soluble content. Most of the galactosamine in these glycoproteins is not present in short ‘mucin type’ oligosaccharides. Chromogranin A is a glycpprotein containing mainly galactose, N-acetylgalactosamine and sialic acid. On the other hand, soluble dopamine β-hydroxylase possesses a high concentration of fucose, mannose and N-acetylglucosamine. This carbohydrate composition is similar to that of the total enzyme described by others, which indicates that the soluble and membrane-bound enzyme have identical carbohydrate moieties. The only glycosaminoglycans (mucopolysaccharides) detected were chondroitin sulphate and heparan sulphate. Glycosaminoglycans are present in a similar concentration in the soluble content and in the membranes of chromaffin granules; however, the relative proportion of heparan sulphate in the membranes is over twice that found in the soluble compartment. Chromaffin granule membranes also contain a relatively high concentration of gangliosides, of which over 90% is hematoside. Glycoprotein biosynthesis was studied in perfused bovine adrenal glands using [ 3H]fucose. This precursor was chosen since it offered the possibility to label preferentially the enzyme dopamine β-hydroxylase with its relatively high concentration of this sugar. The soluble glycoproteins of newly formed chromaffin granules incorporated significant amounts of fucose radioactivity (most of it apparently into dopamine β-hydroxylase), whereas very little labelling of the membranes occurred. These results, which indicate that the soluble and membrane-bound forms of dopamine β-hydroxylase have different synthesis rates, are discussed in connection with the concept that membranes of chromaffin granules are reused for several secretory cycles.