ラット (ビタミンA欠乏系) に投与したカロテノイド種のRBP (レチノール結合蛋白) 代謝に及ぼす影響

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  • Metabolic effects between RBP (retinol binding protein) and Carotenoids fed Vitamin A deficient Rats
  • ラット ビタミン A ケツボウケイ ニ トウヨ シタ カロテノイドシュ ノ RBP レチノール ケツゴウ タンパク タイシャ ニ オヨボス エイキョウ

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Conversion of carotenoids to retinol is due fundamentally to the presence of β-retinylidene residues (β-ionone ring) as end-groups in the carotenoids. It is well known that in some fish xanthophylls such as canthaxanthin, astaxanthin and zeaxanthin, those that generally are not termed as Provitamin A, are converted to retinol by reductive metabolism. However, in mammals, it has not been made clear whether these xanthophylls are converted to retinol or not. This study, based on clinical experiments on rats, was carried out in order to clarify this particular issue.<BR>60 male rats (Sprague Dawly, 3 weeks old) were grown for 21 days on a Vitamin A (V. A) deficient diet prior to the experiment. They were then divided into 10 groups, 2 of which were used as control groups. Each of the remaining 8 groups was fed on a diet containing β-carotene, β-cryptoxanthin, zeaxanthin, lutein, canthaxanthin, astaxanthin, lycopene, or retinyl acetate for 8 day. After the experimental period, the weight ofthe bodies and internal organs were measured for all rats. At the same time, the concentration of carotenoids, retinoids, and retinol binding protein (RBP) in the blood and internal organs was determined. Since retinol binds to RBP in blood with a 1: 1 molar ratio, the concentration of retinol corresponds to that of RBP. Therefore, the concentration of RBP can be a good measure to determine the conversion efficiency of carotenoids to retinol.<BR>The reduced body weight of rats fed on the V.A deficient diet significantly increased for the diet containing β-carotene or β-cryptoxanthin. However, a remarkable increase was not detected for the other carotenoids. The content of carotenoids in the liver was remarkably high for the diet containing lycopene, and was high for the diet containing β-carotene, β-cryptoxanthin, or canthaxanthin. For the astaxanthin-, lutein-or zeaxanthin-group, the carotenoid content in the liver was negligible. The content of retinol in the blood was 4.3 times that of the control group in the β-carotene group, 3.2 times in the β-cryptoxanthin group, 1.1 times in the canthaxanthin group, and 0.8 times in the lycopene group.Remarkable changes were not observed in the other groups. The RBP content in the blood was 4.5 times that of the control group in the β-carotene group, 2.8 times in the β-cryptoxanthin group, 1.2 times in the canthaxanthin group, and 0.8 times in the lycopene group. Marked changes were not observed in the other groups.<BR>Based on all the findings conclusions were made as follows.(1)β-arotene and β-cryptoxanthin are quite easily converted to retinol by oxidative metabolism due to the presence of β-ionone end-groups.(2) Conversion from lycopene to retinol is difficult because of the presence of the ψ-ionone end-group.(3) Conversion from canthaxanthin, astaxanthin, or zeaxanthin to retinol via β-carotene and that from astaxanthin or lutein to 3, 4 d-ehydroretinol via anhydro 3, 4-dehydroretinol are difficult in mammals unless some reductive reagents are included in the diet. In order to confirm the final conclusion, further investigation is necessary, for example, by elongating the period during which the rats are fed the diet containing the xanthophylls.

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