More than 85% of the apoprotein of LDL used
in this research (d = 1.025-1.045) was insoluble in
tetramethylurea (28) and only trace amounts entered
the running gel upon electrophoresis in 7.5%
polyacrylamide containing 8 M urea (4, 28). On
double immunodiffusion, antisera against rat LDL
formed one major arc with VLDL, LDL, and
whole rat serum (Fig. 1). In each case, the major
arcs formed with these three antigens show a
reaction of identity. The identity of this as the B
protein was established by its characteristic position
on both immunodiffusion and immunoelectrophoresis
(11). Antiserum against rat VLDL produced
the B arc as well as three other arcs with
VLDL and two additional arcs with HDL. Two of
these arcs represent VLDL proteins which are also
present in HDL. One of the latter is also found in
the fraction of density greater than 1.21 g/cm ~.