Title : Identity of neutral alpha-glucosidase AB and the glycoprotein processing enzyme glucosidase II
Abstract :
Biochemical and genetic studies
We have previously partially purified, characterized, and chromosomally mapped a human isozyme of alpha-glucosidase which is active at neutral pH. This isozyme appears as a doublet of enzyme activity on native gel electrophoresis and was termed neutral alpha-glucosidase AB
We now report genetic and biochemical evidence that neutral alpha-glucosidase AB is synonymous with the glycoprotein processing enzyme glucosidase II
We have found that a mutant mouse lymphoma line which is deficient in glucosidase II is also deficient in neutral alpha-glucosidase AB , as defined electrophoretically and quantitatively (less than 0.5% of parental)
In contrast, both mutant and parental cell lines exhibited several lysosomal hydrolases which are processed by glucosidase II
We have also further purified the human neutral alpha-glucosidase A component of neutral alpha-glucosidase AB 740-fold from placenta in order to compare its biochemical properties with those described for rat liver and pig kidney glucosidase II
Both glucosidase II and neutral alpha-glucosidase AB are high-molecular mass (greater than 200,000 dalton) anionic glycoproteins which bind to concanavalin A , have a broad pH optima (5.5-8.5), and have a similar Km for maltose (4.8 versus 2.1 mM) and the artificial substrate 4-methylumbelliferyl-alpha-D-glucopyranoside (35 versus 19 microM)
Similar to human neutral alpha-glucosidase AB , purified rat glucosidase II migrates as a doublet of enzyme activity on native gel electrophoresis
Although rat glucosidase II has been reported to have a subunit size of 67 kDa, pig glucosidase II has been found to have a subunit size of 100 kDa, like the 98-kDa major protein in purified human neutral alpha-glucosidase A . Although we have not demonstrated that neutral alpha-glucosidase AB is microsomal nor that it hydrolyzes the natural substrate of glucosidase II , we believe that the genetic evidence is compelling for and the biochemical data consistent with the hypothesis that neutral alpha-glucosidase AB and glucosidase II are synonymous
These and previous results would localize glucosidase II to the long arm of human chromosome II