Glucose biosensors based on direct electron transfer (DET) between two types of PQQ dependent glucose dehydrogenases: soluble (s-PQQ-GDH) and membrane-bound (m-PQQ-GDH) and modified carbon black electrodes were created. The behaviour of both enzymes on carbon paste electrodes was investigated and some differences were elucidated. It was demonstrated that for the achievement of DET between the adsorbed s-PQQ-GDH or m-PQQ-GDH and the carbon paste electrodes particular orientation of the enzymes is required. The stability and the selectivity of the biosensors were studied as well. The calculated inactivation constants for s-PQQ-GDH and m-PQQ-GDH-based biosensors were 0.001 and 0.0001 min(-1), respectively. Furthermore, the possibility to detect lactose concentration in dairy products was shown. (c) 2006 Elsevier Ltd. All rights reserved.