Golgi alpha 1,2-mannosidase I is involved in the N-linked oligosaccharide processing pathway. In this study, two truncated genes encoding for human Golgi alpha 1,2-mannosidase I (hManIA2: amino acids 127-626 and hManIC: amino acids 118-617) were expressed in Escherichia coil to characterize the enzymes. These genes were fused to a 17 protein tag and a histidine tag at the N- and C-terminal ends, respectively, and purified using Co(2+) affinity chromatography. The properties including optimal temperature, optimal pH, and substrate specificity of the purified enzymes were investigated by HPLC using pyridylamino (PA)-labeled oligosaccharides as substrates. The stability of hManIA2 was dependent on the presence of Ca(2+), which was also required for its activity. On the other hand, hManIC was stable in the absence of Ca(2+), even though Ca(2+) was also effective for the activity of hManIC. While the similarity of the amino acid sequences is over 60%, hManIA2 and hManIC showed different substrate specificities particularly toward M9A and M8C. (C) 2011, The Society for Biotechnology, Japan. All rights reserved.