Biochemical Identification

There has been a number of demonstrable instances of cross-culture contamination. This occurs by manipulative errors, and though rare, the introduction of a single foreign cell, with slight growth rate advantage, can be sufficient to swamp the host population. The morphological similarity of mammalian cells in culture, as seen with biology microscopes, prevents the early identification of cross-culture contamination in most cases. Biological identification is basically a problem in reading the cell’s genotype through its phenotype, for only a small fraction of the cell’s genotype is expressed phenotypically. Measurements include cytological, immunological, and electrophoretic enzyme assays. The electrophoretic enzyme assays are simple and direct and may be used for both interspecific and intraspecific contamination problems. Since most, if not all enzymes have species-specific amino acid sequences, it is possible with one or two enzymes to identify definitively the species derivation of a cell line, which can be note through biology microscopes.

Intraspecific identification problems are more difficult in that, in order for the enzyme marker to be useful, it must be polymorphic, and there are only a few human enzymes known at the present time which are both polymorphic and detectable in fibroblasts in culture. There are several methods available for electrophoretic separation of different enzymes, including the use of cellulose strips, Cellogel, and starch gel.

Cytoenzymology

Since the demonstration of phosphatases in tissue sections in 1939, cytohistochemistry has grown so rapidly. The techniques of cytoenzymology consist of chemical reactions produced in cells and tissue sections that yield insoluble end products easily seen with the biology microscopes, and which furnishes morphological expressions of many enzymes and of many chemical components of cells. The reactions of cytoenzymology have special meaning because their chemical principles are understood.

Isozymes are the multiple molecular forms of enzymes that catalyze conversion of the same substrate. In free-living cells and in tissue sections, cytohistochemical enzyme techniques produce reaction products that are the sum total of the different isozymes. The various isozymes have different electrophoretic mobilities that enable them to be individually isolated in gel media by electrophoresis, and to be readily demonstrated by application of histochemical enzyme incubating solutions to the gels.