140 LARS ERNSTER 



DT Diaphorase : properties and functional aspects 



PROPERTIES, AND COMPARISON WITH VITAMIN K REDUCTASE 



In 1958, the occurrence of an abundant diaphorase activity in the 

 sohible cytoplasm of rat hver was detected in our laboratory [13, 14]. The 

 enzyme catalyzed the reduction of 2,6-dichlorophenolindophenol by 

 DPNH and TPNH at equal rates, and therefore we decided to call it 

 " DT diaphorase ". Early this year [15], we reported the partial purification 

 of the enzyme and some of its properties. These can be summarized as 

 follows: (i) The enzyme is a flavoprotein with a very high turnover 

 number, of the order of ten millions. (2) It reacts at equal maximal rates 

 with DPNH and TPNH, but its affinity for TPNH is slightly higher than 

 for DPNH. (3) Besides various dyestuffs and ferricyanide, a number of 

 naphtho- and benzoquinones can serve as electron acceptors, but not 

 vitamin K^, or any long-chain substituted quinones. (4) The enzyme is 

 strongly inhibited by dicoumarol, and the inhibition is not competitive 

 with regard to the electron acceptor. (5) It is inhibited by sulphydryl 

 reagents ; and (6) by thyroxine and related compounds. (7) It is activated 

 by bovine serum albumin, which increases both the maximum velocity of 

 the enzyme and its affinity for its substrates. (8) Although the enzyme is 

 present in both mitochondria and microsomes, it is most abundant in 

 the soluble cytoplasmic fraction. 



Several of these properties resembled those of various bacterial and 

 plant quinone reductases described by Wosilait and associates [16-18],* as 

 well as those of the vitamin K or phylloquinone reductase of Martins [14]. 

 However, DT diaphorase clearly differed from Martins 's enzyme in that it 

 did not react at any appreciable rate with vitamin K^, whereas this com- 

 pound is the only electron acceptor specified in papers published between 

 1954 and 1959 by Martins and collaborators. 



This situation markedly changed a few weeks ago. In a paper which has 

 just appeared, Miirki and Martins [21] now report properties of vitamin K 

 reductase which differ in several important respects from those they 

 previously reported. Moreover the newly reported properties of the enzyme 

 strongly resemble those of DT diaphorase. A brief summary of this develop- 

 ment (which escaped recognition in the Miirki and Martins paper) is shown 

 in Table I. In fact, except for its insensitivity to — SH reagents and to 

 thyroxine, vitamin K reductase now reveals almost identical properties 

 with those of DT diaphorase, and therefore, we are strongly inclined to 

 conclude that the two enzymes are identical. During the last two years, a 

 considerable amount of information has accumulated in our laboratory 



* A similar quinone reductase from dog liver has recently been described by 

 Wosilait [19]. The isolation of a DT diaphorase-like flavoenzyme from brain tissue 

 has been briefly reported by Giuditta and Strecker [20]. 



