Autolysis of glycoproteins in rat kidney lysosomes in vitro. Effects on the isoelectric focusing behaviour of glycoproteins, arylsulphatase and β-glucuronidase

Abstract

1. Rat kidney lysosomal glycoproteins, prelabelled in the N-acetylneuraminic acid and polypeptide portions with N-acetyl[3H]mannosamine and [14C]lysine, or with N-acetyl-[14C]glucosamine, were incubated under various conditions. Autolytic cleavage of labelled N-acetylneuraminic acid and peptide was maximum at pH5.0. 2. N-Acetylneuraminic acid was released more rapidly than peptide during incubation at 37° or 4°C at pH5. p-Nitrophenyloxamic acid, an inhibitor of bacterial neuraminidase (Edmond et al., 1966), inhibited the cleavage of N-acetylneuraminic acid and peptide, and also inhibited cathepsin D activity. 3. Galactono-, mannono-, and glucono-lactone, inhibitors of the corresponding glycosidases, blocked the autolytic cleavage of N-acetyl[14C]glucosamine and protein without inhibiting β-N-acetylhexosaminidase or cathepsin D activity. These findings suggest that the carbohydrate side chains protect the polypeptide portion of the lysosomal glycoproteins against proteolytic attack by lysosomal cathepsins. 4. In electrofocusing experiments, autolysis was minimized by adding 0.1% p-nitrophenyloxamic acid to the media used for extraction and electrofocusing, and by maintaining an alkaline pH (pH8.8–9) during extraction and dialysis. Arylsulphatase occurred in two forms with pI values of 4.4 and 6.4–6.7, and β-glucuronidase in two forms with pI values of 4.4 and 6.1. When [14C]lysine and N-acetyl[3H]mannosamine were given to rats 1.5 and 1 h before killing, 14C and 3H were largely restricted to highly acidic glycoprotein species with pI values of 2.1–5.1. 5. When a lysosomal extract was adjusted to pH5 and incubated at 20°C for 16h and then at 37°C for 1 h before electrofocusing, 32 and 58% of the labelled peptide and N-acetylneuraminic acid was cleaved and the pI values of the labelled glycoproteins were markedly increased. About 80% of the acidic form of arylsulphatase and β-glucuronidase was recovered with the basic form, and the pI of the basic form of both enzymes rose to 7.0. Similar, though less marked changes, were observed when a lysosomal extract was kept at pH5 for 2h at 4°C before electrofocusing. 6. When an acidic lysosomal fraction (pI4.2–4.6) was incubated at pH5 for 2.5h and refocused, 80% of the arylsulphatase now occurred in two forms with pI values of 5 and 6.4. When a basic lysosomal fraction (pI5.8–6.4) was similarly incubated, the pI of arylsulphatase increased from 6.4 to 7.2. The relative increase in pI of arylsulphatases was accompanied by a proportional loss of N-acetylneuraminic acid from the glycoprotein associated with these forms. 7. These experiments show that lysosomal glycoproteins and two representative hydrolases, when exposed to a mildly acidic pH, readily undergo autolytic degradation and their pI values increase. These observations may have a bearing on the origin of the molecular heterogeneity of the lysosomal enzymes.