Dark effects of Hematoporphyrin Derivative on Lactate Dehydrogenase activity and distribution in HeLa cells: Cytochemical evaluation

The effects of hematoporphyrin derivative (HpD) in the dark on the activity and subcellular location of lactate dehydrogenase (LDH) in HeLa cells are evaluated with an original quantitative cytochemical technique. The enzyme activity is visually indicated by diffuse patterns ascribed to loosely-bound, cytoplasm-LDH and by granular formations ascribed to tightly bound, mitochondrial-LDH. The granular pattern has a longer formation lag than the diffuse pattern, since mitochondrial loci are less accessible to the reagents. Observations occasionally and unexpectedly found show that the percentage of cells with granular pattern is highly enhanced in suffering cells. This suggests that the mitochondrial-LDH activity increases as a reaction to stress. In cells that have incorporated HpD for 24 h in the dark, significant changes of LDH activity and distribution occur as many more cells with granular pattern and larger granules are seen, while the diffuse activity significantly decreases. These results point towards: (a) a binding of HpD at the cytoplasm LDH active site or close to it leading to enzyme inactivation; (b) a labilization of the mitochondrial membrane facilitating access to the inner loci by the cytochemical reagents and hence better visualization of the mitochondrial-enzyme; and/or (c) an increased LDH activity ensuing from drug-induced stress.