Physicochemical modifications of lysosomal hydrolases during intracellular transport

Abstract

1. The following fractions were prepared from rat kidney and characterized ultrastructurally, biochemically and enzymically: (a) an ordinary rough microsomal (RM1) fraction; (b) a special rough microsomal (RM2) fraction enriched seven- to nine-fold in acid hydrolases over the homogenate; (c) a smooth microsomal (SM) fraction; (d) a Golgi (GM) fraction enriched 2.5-fold in acid hydrolases and 10-, 15- and 20-fold in sialyltransferase, N-acetyl-lactosamine synthetase and galactosyltransferase respectively; (e) a lysosomal (L) fraction enriched 15- to 23-fold in acid hydrolases. The frequency of Golgi sacs and tubules seen in the electron microscope and the specific activity of the three glycosyltransferases in these fractions increased in the order: RM2<RM1<SM<GM. 2. Five lysosomal hydrolases, acid phosphatase, β-N-acetyl-hexosaminidase, β-galactosidase, β-glucuronidase and arylsulphatase, were characterized in these fractions with respect to (a) solubility on freeze–thawing and (b) electrophoretic mobility in polyacrylamide gels. 3. In the RM2 fraction each of these hydrolases occurred largely or exclusively as a single bound basic form coincident with cationic glycoprotein bands in gels (Goldstone et al., 1973). 4. In the L fraction these hydrolases were present largely as soluble, acidic (anionic) forms. 5. The solubility, electrophoretic heterogeneity and anodic mobility of these hydrolases increased progressively in subcellular fractions in the order: RM2<RM1<SM<GM<L. 6. These findings, together with evidence cited in the text showing that N-acetylneuraminic acid residues are responsible for the solubility and electronegative charge of these acidic forms and incorporation of these residues into the Golgi apparatus, support the following scheme for the biosynthesis of lysosomal enzymes. Each hydrolase is synthesized as a bound basic glycoprotein enzyme in a restricted portion of the rough endoplasmic reticulum. The soluble, acidic forms are generated as the nascent glycoprotein enzymes migrate through the Golgi apparatus through the attachment of sugar sequences containing N-acetylneuraminic acid.