Conversion of DPNH Dehydrogenase to DPNH-cytochrome Reductase by Thiourea

INVESTIGATIONS in this laboratory have shown that the various DPNH-cytochrome reductase preparations from heart and liver mitochondria described in the literature are fragments of the respiratory chain-linked DPNH dehydrogenase produced by the extraction method (acid-ethanol-heat or thermal degradation) used in the isolation of cytochrome reductases 1–5 . By the action of these agents as well as various proteolytic enzymes, urea and under a number of other experimental conditions the purified dehydrogenase is also degraded to lower molecular weight cytochrome-reducing fragments. This conversion involves major changes in many of the molecular and catalytic properties of the enzyme of which the emergence of cytochrome (and DCIP) reductase activity is but one feature. The observations on thermal and proteolytic degradation have been confirmed and extended by Kaniuga 7,8 , who has independently discovered the urea-catalysed transformation of the enzyme. Acidethanol degraded DPNH dehydrogenase is not distinguishable from the preparations of Mahler et al. 9 , de Bernard 10 , and Mackler 11 , while the thermally degraded enzyme is in every respect identical with the preparation isolated by King and Howard 12 of which the major component is the Mahler enzyme 9 . Although these investigations have directed attention to the fact that DPNH dehydrogenase is unstable under a variety of conditions 13 and readily breaks down to fragments with very different properties, very recently yet another DPNH oxidizing preparation derived from heart mitochondria has been described 14 which appears to be a breakdown product of the respiratory chain-linked DPNH dehydrogenase. It is the purpose of this communication to show that, in terms of available evidence, the “DPNH dehydrogenase” of Chapman and Jagannathan 14 is not a naturally occurring enzyme but a fragment produced by the action of thiourea on DPNH dehydrogenase.