Stable ion complexes of vitamin A species (retinol) in solution as detected by mass spectrometry

Mechanisms of retinol (ROL) transport in plasma alternative to that involving the Retinol-Binding Protein (RBP) have been hypothesized after RBP-knockout mice were shown to be viable and fertile. Accordingly, the possibility of a ROL fraction circulating free in plasma has been suggested. In this study, stable complex formation between ROL and the physiological relevant ions Na(+), Li(+), and K(+) was investigated by using mass spectrometry and assessed in regard to the aforementioned alternative mechanism. The ROL-Na complex was found to be kinetically favoured with respect to the ROL-Li and ROL-K complexes. Fragmentation of the [ROL-Na](+) adduct rendered the carbocation [ROL+H-H(2)O](+) (m/z 269) and NaOH as neutral loss. In consonance with these results, it is hypothesized on the possibility of the complex [ROL-Na](+) being an alternative way of ROL transport to cells, as well as an intermediate in cis/trans isomerism.