CONVERSION OF CAROTENOIDS INTO VITAMIN A 



be noted in passing that one claim exists that subcutaneously injected 

 carotene is effective. " * 



A thorough re-examination of the problem by Sexton, Mehl and 

 Deuel, ® ^ has left no doubt as to the ineffectiveness of parenterally 

 administered carotene, irrespective of whether the injection is intra- 

 peritoneal, intravenous, intrasplenic, or intracardiac. In fact after 

 intrasplenic injection of carotene into vitamin A-deficient rats the 

 deficiency symptoms persisted although substantial amounts of carotene 

 reached the liver and were stored there. Sexton et al.^'" pointed out 

 that their results strongly suggested the intestine as the site of con- 

 version. This was the first firm suggestion that the intestine was 

 involved, although it was mooted by Verzar and McDougall in 1936^ • 

 and Wagner and Vermeulen had considered this possibility in whales. ^ ' 

 However, a preliminary in vitro experiment in which Sexton et al. ^ ^ 

 dosed vitamin A-depleted rats with carotene and then removed the 

 intestines and incubated them for 6-24 hours, provided no evidence of 

 vitamin A formation. 



Meanwhile, investigations were being carried out at Liverpool which 

 all pointed to the intestine as the site of conversion. This was implicit 

 in the results of the work of Goodwin, Dewar and Gregory, ^ ^ who 

 could not demonstrate the presence of carotene in either the portal or 

 systemic blood of living sheep and goats even after very high doses of 

 carotene and after by-passing the rumen by feeding the carotene 

 directly into the duodenum via a duodenal cannula. Further, Ball, 

 Glover, Goodwin and Morton, * * and Glover, Goodwin and Morton, " ^ 

 demonstrated the conversion of vitamin A aldehyde (retinene) into 

 vitamin A in the intestinal wall ; this was very significant for, in all 

 probability, retinene is an intermediate in the conversion of ^-carotene 

 into vitamin A. 



It was not surprising then that in vivo reports of the conversion of 

 carotene into vitamin A in the intestinal wall of rats soon appeared 

 first from Liverpool ^ °' ^ ^ then almost immediately afterwards from 

 California. ^ 2- » * Meanwhile, Thompson, Ganguly and Kon ^ 5' ^ « 

 had, using a slightly different technique reached the same conclusion 

 for pigs. Goodwin and Gregory's^' experiments with goats were 

 successfully concluded when they demonstrated a rise in the concen- 

 tration of vitamin A in thoracic Jymph after feeding p-carotene. 

 The failure previously to find an increase in the vitamin A blood 

 plasma level after feeding p-carotene was due to the dynamics of 

 the situation ; the vitamin A produced in the intestinal wall accumu- 

 lated in the thoracic lymph in amounts which were easily detectable ; 

 when this lymph was delivered into the systemic blood stream it was 



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