520 2. ANALOGS OF ENZYME REACTION COMPONENTS 



Accumulation of Intermediates and in Vivo Effects 



Inasmuch as the oxidation of pyruvate requires thiamine-PP, one would 

 expect some accumulation of pyruvate in animals treated with thiamine 

 analogs if pyruvate oxidase is indeed inhibited in vivo. Such has been ob- 

 served in rats with oxythiamine (Frohman and Day, 1949; Gubler, 1961) 

 and pyrithiamine (de Caro et al., 1954). For example, rats injected intra- 

 peritoneally with 150 //g oxythiamine show an elevation in blood pyruvate 

 of 1.3 to 5.1 mg%; blood lactate also increases from 11.5 to 42.8 mg%. In 

 mice, pyrithiamine raises the blood pyruvate somewhat but oxythiamine 

 has no effect (de Caro et al., 1956), possibly indicating a species difference 

 since in rats oxythiamine is more effective than pyrithiamine (Gubler, 

 1961). The administration of oxythiamine to dogs at 6 mg/kg in three 

 doses leads to a marked rise in blood pyruvate (0.4 to 5.7 mg%) and thia- 

 mine is able to counteract this effectively (Wilson et al., 1962). Simultan- 

 eously there is a severe fall in liver glycogen (13 to 0.4 mg/g). Rats and 

 cats respond similarly but are less sensitive. Growth of Neurospora in the 

 presence of oxythiamine is accompanied by pyruvate accumulation and a 

 simultaneous reduction in pyruvate decarboxylase activity is demonstrable 

 (Sankar, 1958). Administration of increasing amounts of thiamine partially 

 or completely counteracts these effects on pyruvate levels, in all instances 

 where it has been tested. 



We shall now turn to evidence of enzyme inhibition in the tissues of 

 analog-treated animals. It may be calculated from the data of Von Holt et al. 

 (1955) that feeding pjTithiamine to rats at 10 mg/kg for 7-12 days results 

 in some 63% reduction of pyruvate oxidation in liver homogenates. A 

 thorough investigation of the changing patterns of keto acid oxidation in 

 deficient and analog-treated rats has been made by Gubler (1958, 1961); 

 his results are summarized in Table 2-32. It is seen that the oxidation of 

 pyruvate is more sensitive than that of «-ketoglutarate to both dietary 

 deficiency and the analogs; this could relate to different displacing rates 

 in the two oxidases, or to different dependencies of enzyme activity on 

 thiamine-PP level. Oxidation of /5-keto acids, as expected, is not affected. 

 The effects of oxythiamine and pyrithiamine are roughly the same on all 

 tissues, with the exception of brain in which pyrithiamine is more effective. 

 The reason for this is not understood — it would seem unlikely that oxy- 

 thiamine is unable to penetrate the blood-brain barrier — but it may be 

 correlated with the fact that only pyrithiamine is able to produce poly- 

 neuritis in rats. Another difference between these two analogs lies in the 

 ability of thiamine-PP added in vitro to the mitochondrial suspensions to 

 counteract the depression of pyruvate oxidation. The loss of activity from 

 dietary deficiency of thiamine is readily reversed by adding thiamine-PP, 

 as anticipated; the loss due to pyrithiamine is surprisingly weU reversed 

 (to about 90% of the control values in brain and kidney); the loss due to 



