Expression of aspartylglucosaminidase in human tissues from normal individuals and aspartylglucosaminuria patients.

Aspartylglucosaminidase (AGA: E.C. 3.5.1.26) is a lysosomal amidase that hydrolyzes the N-acetylglucosamine-asparagine linkage as one of the final steps in the breakdown of glycoproteins. Deficiency of this enzyme results in aspartylglucosaminuria (AGU), an inherited lysosomal storage disease. In an attempt to establish the tissue-specific expression of AGA in normal individuals and in AGU patients, we adapted biochemical and immunohistochemical techniques to analyze AGA polypeptides in human cells and tissues. The biochemical analysis revealed the existence of alpha- and beta-subunit structures of AGA in all tissues. Immunohistochemical staining demonstrated a cell specificity in the distribution of AGA: immunoreactivity was strongest in hepatocytes, pyramidal cells in the cerebral cortex, and proximal tubule cells in the kidney. In tissues from AGU patients, AGA immunoreactivity could be detected in hepatocytes and in proximal tubule cells but not in the pyramidal cells. The regulation of the expression of AGA was approached by analyzing the transcript levels and the methylation of the AGA gene. Both heavy methylation of the AGA gene and the constant level of AGA mRNA were typical of a "house-hold" type of enzyme that can be found in small quantities in all tissues. This was in contrast to the variability of the amount of AGA polypeptides observed in different cells and tissues, suggesting that the expression of AGA is regulated not at the transcriptional but rather at the translational level.