ABSTRACT:We have identified an error in the published integral form of the modified Michaelis-Menten equation that accounts for endogenous substrate production. The correct solution is presented and the error in both the substrate concentration, S, and the kinetic parameters V m , K m , and R resulting from the incorrect solution was characterized. The incorrect integral form resulted in substrate concentration errors as high as 50% resulting in 7-50% error in kinetic parameter estimates. To better reflect experimental scenarios, noise containing substrate depletion data were analyzed by both the incorrect and correct integral equations. While both equations resulted in identical fits to substrate depletion data, the final estimates of V m , K m , and R were different and K m and R estimates from the incorrect integral equation deviated substantially from the actual values. Another observation was that at R ¼ 0, the incorrect integral equation reduced to the correct form of the Michaelis-Menten equation. We believe this combination of excellent fits to experimental data, albeit with incorrect kinetic parameter estimates, and the reduction to the Michaelis-Menten equation at R ¼ 0 is primarily responsible for the incorrectness to go unnoticed. However, the resulting error in kinetic parameter estimates will lead to incorrect biological interpretation and we urge the use of the correct integral form presented in this study. Biotechnol. Bioeng. 2011;108: 2499-2503. ß 2011 Wiley Periodicals, Inc. KEYWORDS: Michaelis-Menten equation; nonlinear parameter estimation; endogenous substrate production; progress curve analysis; enzyme kinetics Progress curve analysis is an attractive method of analyzing biological systems as kinetic parameter determination from a single substrate/product concentration versus time data set is possible (Duggleby, 1991(Duggleby, , 2001Robinson, 1998;Robinson and Tiedje, 1983). The Michaelis-Menten equation continues to be the most widely used equation to describe a variety of biological systems and substantial advances have been made for robust analysis of progress curve data using the Michaelis-Menten equation. For instance, parameter correlation, which can impact final kinetic parameter estimates has been well characterized (Ellis et al., 1996;Robinson, 1985Robinson, , 1998 and recommendations for experimental design have been made to enable robust kinetic parameter estimation (Duggleby, 1995(Duggleby, , 2001Holmberg, 1982;Liu and Zachara, 2001;Robinson, 1985Robinson, , 1998Robinson and Tiedje, 1983). Furthermore, kinetic parameter estimation using an explicit closed form solution of the Michaelis-Menten equation has been demonstrated (Goudar et al., 1999(Goudar et al., , 2004) which simplifies analysis since iterative computation of the substrate/product concentration is eliminated.However, not all biological systems can be described by the Michaelis-Menten equation and one class of exceptions includes systems such as sediments, sludge from anaerobic digestors, and rumen fluid, among others...