Radioimmunoassay of human plasma apolipoprotein A-1: pretreatment of plasma with guanidine hydrochloride.

A sensitive and specific double antibody radioimmunoassay for apolipoprotein A-1 was first reported by Schonfeld and Pfleger (1). Their procedure required delipidation of the serum since direct radioimmunoassay of apolipoprotein A-1 in untreated sera resulted in much lower values than those obtained from the corresponding delipidated samples. In an attempt to find a rapid and simple procedure for radioimmunoassay of apolipoprotein A-1 without using organic solvents, the serum was subjected to various physical and chemical treatments which disrupt or alter high density lipoproteins (HDL). Heating at 52 C for 3 h and the treatments with 0.1% Triton X-100, 0.05 M sodium dodecyl sulfate (SDS), and 8 M urea resulted in an increased immunoreactivity of apolipoprotein A-1; the reactivity, however, was much lower than that obtained from delipidated samples. Treatment with 6 M guanidine hydrochloride for 3 h at 37 C prior to radioimmunoassay resulted in a maximal increase of apolipoprotein A-1 immunoreactivity comparable to that obtained with delipidated samples. This pretreatment permits a large number of samples to be assayed with complete recovery of apolipoprotein A-1.