1,4-Dideoxy-1,4-imino-D-mannitol inhibits glycoprotein processing and mannosidase

1,4-Dideoxy-1,4-imino-D-mannitol (DIM) was synthesized chemically from benzyl-alpha-D-mannopyranoside Fleet et al (1984) J. Chem. Soc. Chem. Commun., 1240-1241], and was tested in vitro as an inhibitor of various alpha-mannosidases and in cell culture as an inhibitor of glycoprotein processing. DIM proved to be an effective inhibitor of jack bean alpha-mannosidase, with 50% inhibition requiring 25 to 50 ng/ml inhibitor. It also inhibited lysosomal alpha-mannosidase, but in this case 50% inhibition required about 1 to 2 micrograms/ml. In both cases, the inhibition was of the competitive type when p-nitrophenyl-alpha-D-mannopyranoside was used as the substrate. The inhibition was better at higher pH values, suggesting that DIM was more effective when the nitrogen in the ring was in the unprotonated form. In addition, rat liver processing mannosidase I was also inhibited by DIM as measured by the release of 3H]mannose from 3H]mannose-labeled Man9GlcNAc. Glycoprotein processing was examined in influenza virus-infected MDCK cells. Infected cells were incubated in various concentrations of DIM and labeled with 2-3H]mannose. Viral and cell pellets were digested with Pronase and glycopeptides were isolated by gel filtration on columns of Bio-Gel P-4. The glycopeptides were then treated with endoglucosaminidase H (Endo H) and rechromatographed on the Bio-Gel column in order to distinguish complex from high-mannose structures. As the DIM concentration in the medium was raised, more and more of the 3H]mannose was incorporated into high-mannose oligosaccharides, and less and less radioactivity was in the complex chains. Most of the Endo H-released oligosaccharides induced by DIM were of the Man9GlcNAc structure, as determined by gel filtration, HPLC, and digestion by alpha-mannosidase. Thus, DIM also appears to inhibit mannosidase I in cell culture. However, about 15% of the Endo H-released oligosaccharides appear to be hybrid types of oligosaccharides, suggesting that DIM may also inhibit mannosidase II.