The absorption and transport of vitamin D in the body

The study of the absorption of vitamin D and its further transformation into the body is important to clarify the mechanism of both physiological and pathological effects of the drug. A number of authors (Thompson a. oth., 1966) found that vitamin D is absorbed in the small intestine. D. Hollander (1971) estimates that more than intensively absorbed vitamin D2 in the segments of duodenal and jejunal guts. Slow absorption was observed in segments of the ileum. At intravenous introduction of radioactive vitamin D latter is found in the mucous tissue of the small intestine, in the future arrives in the lymphatic system of the intestine, where 80% of it falls on hilomikronov faction. Thus the necessity of the presence of bile for the absorption of vitamin D. it is Established that this vitamin is absorbed from the intestines in the form micelles, containing bile salts, fatty acids and monoglycerides and transferred to the lymphatic system. D. Schachter, E. Dowdle (1960), studying the impact of bile on the absorption of vitamin D in rats with drainage biliary fistula, found that these animals were learned on average 38% of the administered drug, while controlling about 50%. Adding to the food taurocholate sodium increased its absorption in the gut in two times.
In the transport of this drug play an important role chylomicrons and blood lipoproteins that proved N. F. De Luca (1967) and his staff with radioactive vitamin D. This gave grounds to believe that initially the vitamin D comes into the lymphatic system is associated with chylomicrons and blood lipoproteins. Then penetrates into the liver, where it connects with aghabullogue faction (Neville, De Luca, 1966). Separated proteins of blood through elektroforeza on polyacrylamide disks, N. F. De Luca found that vitamin D in the blood rather strongly associated with lipoprotein and L2-globulin fractions. The author believes that the L2-globulin is a carrier of vitamin D and, apparently, plays a significant role in transmission and metabolism.
The further way of transportation of vitamin D in the body is associated with a number of complex transformations. Because of the small intestine is the site of the intake where then is manifested main function of its active form (1,25-DGHC), it is possible to speak about the "circulation" of vitamin D. Following the exchange scheme and the "circuit" of vitamin D2 (Fig. 4), which shows that the absorption of vitamin D2, received from food, occurs in the small intestine, mainly in the duodenum. Then the drug through the lymphatic system enters the blood stream, where it binds with chylomicrons, lipoproteins and L2-globulin fraction. Similar developments are occurring with vitamin D3 is produced in the skin of provitamin D3 under the influence of UV irradiation.
Then the connection with the current blood enters the liver, where under the influence of 25-hydroxylase to the carbon in the position 25 in the side chain of the molecule of vitamin joins one hydroxyl group with the formation of the 25-GCC. Last enters the blood, and then in the kidneys, where it is further oxidation to carbon in position 1 (ring (A) with the participation of the 1-hydroxylase joins the second hydroxyl group with the formation of 1,25-DGHC.
As a result of oxidation of vitamin D in the liver and kidneys to the molecule joined by two oxygen atoms and molecular weight increases with 384 to 416. These relatively small changes in the structure of 4 - 6 times increase physiological activity of the vitamin. In the future, a large part of 1,25-dihydroxytoluene-civerolo through the blood of the kidneys enters the small intestine (12 duodenal ulcer), is the main purpose of vitamin D absorption of calcium. The rest of 1,25-DGHC blood is delivered in various tissues and organs.
From the presented scheme "circuit" vitamin D (Fig. 4) shows that in the suction him in the gut, oxidation in the liver and kidneys in the blood are all formed its metabolites (vitamin D3, 25 MCC, 1,25-DGHC).
When sufficient and excessive intake of vitamin D3 part of it and the newly formed 25 MCC, bypassing the liver and kidneys, penetrates into the subcutaneous fat layer, muscles and other organs which play function depot vitamin. As for 1,25-dihydroxycholecalciferol - the main metabolite of vitamin D, it accumulates in such target tissues, as mucous membrane of the intestine, bone, breast during lactation.