Determination of beta-carotene and its cis isomers in serum.

All-trans-beta-carotene was resolved from its cis isomers in human serum by reversed-phase "high-performance" liquid chromatography. Absorption spectra of the cis peak suggested that 13-cis-beta-carotene was the predominant cis isomer. Analyses and recovery studies of fresh and stored sera eliminated the possibility that isomerization had occurred in samples during handling or storage. The average analytical recovery was 101.9% for standards of the all-trans-, 9-cis-, and 13-cis-beta-carotene compounds in pooled serum samples. We also demonstrated that cis isomers had not formed after the blood was drawn and that cis isomers of beta-carotene are present at significant concentrations in the human circulation.