Serum albumins are the major site for in vivo formation of hapten-carrier protein adducts in plasma from humans and guinea-pigs exposed to type-1 allergy inducing hexahydrophthalic anhydride.

Organic acid anhydrides (OAAs) are highly allergenic compounds used in the chemical industry. The OAAs probably act as haptens but the proteins that form conjugates with OAAs in vivo are still unknown. Conjugates between the anhydrides and serum albumins (SAs) have routinely been used when testing for OAA-specific antibodies. However, the use of SA as the carrier-protein in these tests has never been evaluated. The aim of this study was to identify major and also immunologically relevant protein conjugates of a particularly sensitizing OAA, hexahydrophthalic anhydride (HHPA), in plasma. Plasma was obtained from a HHPA-exposed worker, from a guinea-pig (GP) exposed to HHPA in an exposure chamber for 2 weeks (8 h/day, 5 days/week) and from a GP exposed once, nose-only, to tritium-labelled HHPA for 8 h. The plasma was fractionated using ion exchange chromatography and gel filtration. These fractions and also aliquots of unfractioned plasma were hydrolysed, derivatized and analysed for anhydride adduct content using gas chromatography-mass spectrometry. Further, plasma from the tritium labelled HHPA-exposed GP was separated by SDS gel electrophoresis and analysed by autoradiography. In addition, immunologically relevant proteins were identified through specific IgE and IgG immunoblottings using sera from exposed workers. For humans > 85% and for GPs > 74% of the HHPA-adducts coeluted with SA in plasma. Autoradiography of GP-plasma shows a single 66 kDa protein that binds HHPA. IgE immunoblotting shows a major 66 kDa and a minor 28 kDa protein which could be inhibited by HHPA-SA conjugate. IgG immunoblotting showed a major 66 kDa protein and several minor protein bands. This study shows SA to be the major protein in plasma that forms adducts in vivo with HHPA. The results also show that in an in vitro synthesized HHPA plasma protein conjugate, HHPA-specific IgE and IgG antibodies bind preferably to the SA.