Specificity studies on alpha-mannosidases using oligosaccharides from mannosidosis urine as substrates

Area of Science:

• Biochemistry
• Enzymology
• Glycobiology

Background:

• Alpha-mannosidases are crucial enzymes involved in glycoprotein metabolism.
• Deficiencies in alpha-mannosidase activity lead to lysosomal storage diseases like mannosidosis.
• Understanding enzyme specificity is key to diagnosing and potentially treating these disorders.

Purpose of the Study:

• To investigate the substrate specificities of human and bovine acidic alpha-mannosidases.
• To compare the activity of different alpha-mannosidase isoenzymes (A, B2, C) using natural oligosaccharide substrates.

Main Methods:

• Oligosaccharides with terminal alpha(1→2), alpha(1→3), and alpha(1→6)-linked mannose residues were isolated from human and bovine mannosidosis urines.
• These oligosaccharides were used as substrates to assay the activity of purified human and bovine liver acidic alpha-mannosidases.
• Enzyme kinetics, including apparent Km, and pH optima were determined.

Main Results:

• Both human and bovine acidic alpha-mannosidases released all tested alpha-linked mannose residues, with highest activity on the smallest oligosaccharide substrates.
• Enzyme A exhibited lower activity than alpha-mannosidase B2, despite having similar apparent Km values.
• Human acidic alpha-mannosidases demonstrated greater activity on human-derived mannosidosis oligosaccharides compared to bovine-derived ones. Human alpha-mannosidase C showed activity only at acidic pH.

Conclusions:

• Acidic alpha-mannosidases from human and bovine liver exhibit broad specificity for alpha-linked mannose residues in oligosaccharides.
• Enzyme source and specific isoenzyme type significantly influence alpha-mannosidase activity and substrate preference.
• These findings contribute to the understanding of alpha-mannosidase function in health and disease, particularly in mannosidosis.