Tunichromes, vanadium, and vacuolated blood cells in tunicates

Abstract

Summary This review covers three aspects of tunicate blood: its yellow/green pigmentation, vanadium content, and vacuolated blood cells. The yellow/green pigmentation is due to a class of organic compounds called tunichromes. A systematic procedure of isolation, purification, and structure determination carried out on blood of Ascidia nigra yielded a family of tunichromes called TC An-i, i = 1, 2, 3. Tunichrome An-1, for example, has the formula C₂₆H₂₅N₃O₁₁, moleculer weight 545 dalton; it has a central triglycyl moiety to which three phenolic groups are attached. An assay for free tunichrome has been developed leading to the discovery of such compounds in four additional species. Tunichromes may be involved in vanadium binding, since a ground blood cell pellet placed on a Sephadex LH–20 column and eluted with increasingly polor solvents yields a green band that contains tunichromes and vanadium. Additionally, there is in vitro evidence for vanadium/tunichrome complexes of definite stoichiometry. However, definitive proof of a role for tunichrome in vanadium accumulation in tunicates has not yet been established. Vanadium occurs in several different cells, depending on species of animals. The best evidence implicates at least three types of vacuolated cells as vanadium repositories: amoebocytes, signet ring cells, and morula cells. In some species, A. ceratodes for example, most vanadium is found in grayish colored signet ring cells whereas most of the free tunichrome is found in yellow/green morula cells. Ultrastructural X-ray microprobe studies show that the accumulated vanadium is mainly associated with intracellular membranes and granules. Both these structures also contain high concentrations of sulfur. The release of protons and vanadium from such a hydrophobic region of the cell upon cytolysis in distilled water could account for the acidity of the resulting lysate. AH these factors—pigmentation, vanadium, and vacuolated cells—are present in ova and developing embryos. It is suggested that the functions of these three factors are important for an as yet undetermined aspect of ascidian development.