Involved in the intracellular degradation of the cyclic tetrasaccharide cyclobis-(1-6)-alpha-nigerosyl (CNN) formed extracellularly from starch. Catalyzes the hydrolysis of alpha-1,6-glucosidic linkage from the non-reducing end of isomaltose to yield beta-D-glucose and D-glucose. Can also act on panose and isomaltotriose at a lower rate. It displays low or no activity toward CNN and the general GH15 enzyme substrates such as maltose, soluble starch or dextran. H2O + isomaltose = beta-D-glucose + D-glucose kcat is 11.4 sec(-1) for isomaltose as substrate. kcat is 1.17 sec(-1) for panose as substrate. kcat is 1.07 sec(-1) for isomaltotriose as substrate. Optimum pH is 6.7. Stable between pH 6.5 and 11.2. Optimum temperature is under 35 degrees Celsius. Belongs to the glycosyl hydrolase 15 family.