Comparison of the structure of key variants of microcystin to vasopressin.

Microcystins (MCs) are cyclic heptapeptide compounds [where X(2) (position 2) and Z(4) (position 4) are variable l-aminoacids] produced by cyanobacteria and responsible for severe liver damage in animals ingesting acute doses of the toxic compounds. Certain variants of microcystins are more toxic than others, the differences being commonly ascribed to the hydrophobic nature of the variant. Microcystin-LR (MCLR) [X = l-leucine (L); Z = l-arginine (R); R1 = R2 = CH(3)] is the most toxic of all the microcystins investigated to date. This study investigates the similarity of the structures of MCLR and selected MC variants to the liver specific hormone vasopressin. Structures were compiled in HyperChem(®) (professional version 5.1). Initial comparisons of the MCLR and vasopressin indicated comparable volumes, surface areas and masses. Further studies using RMS overlays show that the microcystin derivative MCLR(Dha(7)) is comparably similar to vasopressin in terms of tertiary structure.