The mechanism of accumulation of high levels of vanadium by ascidians from seawater: biophysical approaches to a remarkable phenomenon.

Abstract

Abstract High levels of vanadium were found by mere chance in the blood cells of an ascidian (tunicates, known as sea squirts) by the German chemist M. Henze in 1911 (1). Subsequently, many investigators revealed that ascidians are the only organisms in the animal kingdom able to accumulate vanadium at high concentrations. We reexamined the vanadium content of several tissues from 20 ascidian species, employing the extremely sensitive method of neutron-activation analysis for the quantification of this metal. The highest concentration of 350 mM vanadium was found in the blood cells of A. gemmata belonging to the suborder Phlebobranchia. Ascidian blood cells have been classified morphologically as being 9 to 11 types. Morula cells, one type of vacuolated cell, have been thought to be the so-called vanadocytes that might be involved in the accumulation of vanadium. Our combination techniques of cell fractionation and neutron-activation analysis revealed that signet ring cells, another type of vacuolated cell, are the true vanadocytes rather than the morula cells. Recently, we generated a monoclonal antibody which reacted with the signet ring cells. On the other hand, the oxidation state of vanadium in the blood cells has been a subject of controversy. ESR measurements of the oxidation state in the fractionated blood cells revealed that the vanadocytes contained vanadium, 97.6% of which was in the +3 oxidation state with the rest being in the +4 oxidation state. Applying ESR spectrometry to the measurement of pH values, we confirmed that the vanadocytes in A. gemmata had a low pH value of 2.42 while these cells contained the highest level of vanadium, 347.2 mM. Furthermore, Raman spectrometry demonstrated the coexistence of sulfate and vanadium in the blood cells of A. gemmata in the ratio of 1.5:1. We tried to shed some light on the basic question of the time at which the accumulation of vanadium commences during embryogenesis using neutron-activation analysis. We found that amounts of vanadium per individual increase dramatically two weeks after fertilization, and by two months the amount accumulated in larvae is about 600, 000 times greater than that in the unfertilized eggs of A. gemmata. A vanadocyte-specific antigen, recognized by a monoclonal antibody specific to the signet ring cells, first appeared in the body wall coinciding with the significant accumulation of vanadium. One and a half months after metamorphosis, the signet ring cells appeared for the first time and were specifically recognized by this antibody. These results mean that the accumulation of vanadium coincides with the differentiation of the vanadocytes during the development of ascidians. The function of the high levels of vanadium in ascidian blood cells remains, however, to be explained.