Reaction rate studies of glucose-6-phosphate dehydrogenase in rat tracheal epithelium; the effect of section thickness.

The reaction velocity of glucose-6-phosphate dehydrogenase has been quantified by continuous monitoring on a Vickers microdensitometer of the reaction product as it formed in sections of different thickness of rat tracheal epithelium. Reaction velocity was directly proportional to section thickness when either tetranitro BT or neotetrazolium was used as the final acceptor; the rate was the same with each tetrazolium salt. However, the amount of formazan deposited in a given time was not proportional to section thickness. When tetranitro BT was employed the reaction became non-linear in the thicker sections due to the inability of the instrument to record beyond a certain absorbance value. Using neotetrazolium a lag phase, due to the failure to overcome the critical supersaturation level of the formazan, preceded the linear response. The duration of this phase decreased as section thickness increased. The implications of these findings on studies using conventional "end point" methods of measurement are discussed.