Characterization of the Isozymes of alpha-Mannosidase Located in the Cell Wall, Protein Bodies, and Endoplasmic Reticulum of Phaseolus vulgaris Cotyledons

Cotyledons of maturing Phaseolus vulgaris seeds contain three isozymes of α-mannosidase which can be separated by isoelectrofocusing. They have isoelectric points of 5.3, 5.8, and 6.5 to 7.5 and were named I, II, and III in order of ascending pI. All three had an acid pH optimum (4.5) and required Zn²⁺ for maximal activity. Isozymes I and II were present in the protein bodies. Together they accounted for 85% of the total activity. Isozyme III was essentially absent from isolated protoplasts but could be extracted from isolated cell walls. All three isozymes were also found to be associated with the endoplasmic reticulum, and the proportion of the total activity in this fraction decreased from 20% in immature cotyledons to 6% in mature cotyledons. The results are interpreted as evidence that newly synthesized α-mannosidase is sequestered in the lumen of the ER prior to its transport to the protein bodies or the cell wall.