EVOLUTIONARY IMPORTANCE FOR THE MEMBRANE ENHANCEMENT OF THE PRODUCTION OF VITAMIN-D3 IN THE SKIN OF POIKILOTHERMIC ANIMALS

The photoproduction of vitamin D in the skin was essential for the evo lutionary development of terrestrial vertebrates. During exposure to s unlight, previtamin D-3 formed in the skin is isomerized to vitamin D- 3 (calciol) by a temperature-dependent process. Since early land verte brates were poikilothermic, the relatively slow conversion of previtam in D-3 to vitamin D-3 at ambient temperature put them at serious risk for developing vitamin D deficiency, thus leading to a poorly minerali zed skeleton that could have ultimately halted further evolutionary de velopment of vertebrates on land. We evaluated the rate of isomerizati on of previtamin D-3 to vitamin D-3 in the skin of iguanas and found t he isomerization rate was enhanced by 1100% and 1700% at 25 degrees C and 5 degrees C, respectively. It is likely that the membrane entrapme nt of previtamin D-3 in its s-cis,s-cis; conformation is responsible f or the markedly enhanced conversion of previtamin D-3 to vitamin D-3. The membrane-enhanced production of vitamin D-3 ensures the critical s upply of vitamin D-3 to poikilothermic animals such as iguanas.