The cell is an extremely complex environment, notably crowded and confining. One third of the cell interior is occupied by biomolecules of all sizes, from a few Angstroems to tens of nanometers in size, membrane-bound organelles and a dense meshwork of cytoskeletal structures that form the mechanical backbone of the cell [1]. This macromolecular crowding affects conformational dynamics and thermodynamics of proteins, and consequently related reaction equilibria. It might significantly decrease the activity of diffusion-limited enzyme kinetics by slowing diffusion and influence the formation of substrate-enzyme complexes, for instance. In this contribution, we investigate the effect of macromolecular crowding on the biocatalytic activity of lactate dehydrogenase (LDH). For this purpose, we use different crowders such as Ficoll PM70, PEG 6000 and PEG 35000 to reduce the volume available for enzymatic transformations. The crowder concentration and the interactions between different components of the system will thus modulate the observed reaction rates. The enzyme lactate dehydrogenase carries the interconversion of pyruvate to lactate and NADH to NAD+, which allows measuring fluorescence changes along the reaction. The experimental data treatement is combined with Brownian dynamics simulations (LAMMPS) of coarse-grained models of interacting molecular species. The extracted kinetic parameters will be discussed in relation with the crowder size and its solution properties.