Relationship between the acid phosphatases of the Kurloff body and the major 30–35 kDa glycoproteins of the Kurloff cell

Abstract

This study deals with the acid phosphatase (AcPase) of the Kurloff body (KB), a large (10 μm diameter) periodic acid-Schiff-positive lysosomal inclusion body present in Kurloff cells (KC). KC AcPase were extracted, with similar yields, either with non-ionic detergent solution or after Dounce homogenization in low ionic strength buffer suggesting that they mainly correspond to hydrosoluble AcPase. After DEAE-cellulose chromatography of such crude Dounce-extracts, 97% of KC AcPase activity was recovered in the unbound glycoprotein fraction (peak I). The main protein content consisted of, as testified by SDS-PAGE analysis, major KC glycoproteins of 30–35 kDa. Thus, KC AcPase, and particularly sialoAcPase, may be assumed to correspond to these N-glycosylproteins among which the presence of α(2,6) sialoglycoproteins was previously established. Following electrophoresis on a 4–15% gradient native polyacrylamide gel or isoelectric focusing, the two size populations (200 kDa and 500 kDa) and up to 20 isoforms of KC AcPase were respectively detected by zymography in peak I. After Clostridium-derived sialidase digestion of peak I, the highly active bands observed at pH 3.5 – 5.2 disappeared. These zymographic patterns were similar to those obtained with crude extracts. After Concanavalin A (ConA)-Sepharose chromatography of peak I, the single ConA-bound glucosamine-labelled peak, eluted at 200 methyl-α- d-mannopyranose, contained the AcPase activity while the ConA-unbound peak was devoid of any acid phosphatase activity. After SDS-PAGE analysis, the ConA-bound fraction appeared to correspond only to a single broad protein band in the 30–35 kDa zone. The Sambucus nigra agglutinin-reactivity of the latter band confirmed the α(2,6) sialylation pattern of KC AcPase. Moreover, their strong binding to ConA-Sepharose suggests that they could only correspond to the hybrid type of the N-glycosylproteins, since they could not correspond to the oligomannosidic type considering the absence of Galanthus nivalis agglutinin-positive structures.