X-ray observations of blazars associated with optically violently variable
(OVV) quasars put strong constraints on the e(+)e(-) pair content of radio-
loud quasar jets. From those observations, we infer that jets in quasars co
ntain many more e+e- pairs than protons, but dynamically are still dominate
d by protons. In particular, we show that pure e(+)e(-) jet models can be e
xcluded, since they overpredict soft X-radiation; likewise, pure proton-ele
ctron jets can be excluded, since they predict too weak nonthermal X-radiat
ion. An intermediate case is viable. We demonstrate that jets that are init
ially proton-electron ("protojets") can be pair-loaded via interaction with
100-300 keV photons produced in hot accretion disc coronae, likely to exis
t in active galactic nuclei in general. If the coronal radiation is powered
by magnetic flares, the pair loading is expected to be nonuniform and nona
xisymmetric. Together with radiation drag, this leads to velocity and densi
ty perturbations in a jet and the formation of shocks, where the pairs are
accelerated. Such a scenario can explain rapid (timescale of similar to a d
ay) variability observed in OVV quasars.