Problems in the bioassay of products from cultured HEK cells: plasminogen activator

For research and commercial purposes, the production of materials from cultured cells is progressively becoming routine. Measuring products in cell culture medium, however, is not always straight-forward even when the most appropriate product assays are utilized. A researcher interested in a particular cell product does not intend to spend time developing or refining existing assays. Instead, he would prefer using techniques and standards reported in the literature. Evaluations of enzyme activity using purified standards or control products may inadvertently lead to the assumption that the same procedures will give reproducible results when cell culture medium is assayed for the product. This report describes the selection of commercially available lots of human embryonic kidney cells and the activity of the plasminogen activator (PA) produced by these cells. PA activity was measured by the fibrin plate assay. The problems of comparing activity in conditioned culture medium with that of purified standard preparations are presented. Factors contributing to non-linearity in dose response curves, inconsistencies in activity in replicate flask cultures, and variations in repeated assays after sample storage are considered. Sample handling procedures and alternate assay systems are discussed.