Simultaneous determination of creatine and creatinine using amperometric biosensors Raluca-Ioana Stefan a, *, Rahel Girmai Bokretsion a , Jacobus F. van Staden a , Hassan Y. Aboul-Enein b a Department of Chemistry, University of Pretoria, 0002 Pretoria, South Africa b Pharmaceutical Analysis and Drug Development Laboratory, Biological and Medical Research Department (MBC-03-65), King Faisal Specialist Hospital and Research Centre, P.O. Box 3354, Riyadh 11211, Saudi Arabia Received 16 January 2003; received in revised form 18 March 2003; accepted 19 March 2003 Abstract In order to determine creatine and creatinine amperometric biosensors were proposed. A bienzymatic biosensor based on creatinase (CI) and sarcosine oxidase (SO) was used for the assay of creatine and a trienzymatic biosensor based on CI, SO and creatininase (CA) for the assay of creatinine. The linear concentration ranges are of pmol l 1 to nmol l 1 magnitude order, with very low limits of detection. The biosensors proved high reliability for determination of creatine and creatinine as raw material, and in the pharmaceutical formulation. # 2003 Elsevier B.V. All rights reserved. Keywords: Creatine; Creatinine; Amperometric biosensors; Creatininase; Creatinase; Sorcosine oxidase 1. Introduction Creatinine is a final product of creatine meta- bolism in mammals. The measurement of creatine and creatinine in blood is important in clinical analyses, because creatinine is important in mon- itoring of kidney function while creatine is a valuable index of muscle damage. The physiologi- cally normal concentration range for creatinine and creatine is below 0.14 mmol l 1 in serum, but during kidney dysfunction or muscle disorder their concentrations may raise up to a value higher than 1 mmol l 1 [1]. Also, creatine has recently great popularity as ergogenic aid. This popularity is based on the fact that creatine is converted to phosphocreatine in muscle in a reversible reaction with adenosine triphosphate (ATP). When muscle contractions deplete the immediate supply of ATP, the phosphocreatine can rephosphorylate adeno- sine diphosphate (ADP) to replenish the supply of ATP. Creatinine is the means by which creatine and phosphocreatine are excreted from the body and this occurs at rates of 0.016 and 0.03 per day for creatine and phosphocreatine, respectively. The amount of creatinine in the urine is propor- tional to the amount of creatine and creatine * Corresponding author. Fax:  /27-12-362-5297. E-mail address: raluca.stefan@chem.up.ac.za (R.-I. Stefan). Talanta 60 (2003) 1223  /1228 www.elsevier.com/locate/talanta 0039-9140/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-9140(03)00230-3