  We revisit the charge-regulation mechanism of macro-ions and apply it to
mobile macro-ions in a bathing salt solution. In particular, we examine the
effects of correlation between various adsorption/desorption sites and analyze
the collective behavior in terms of the solution effective screening
properties. We show that such a behavior can be quantified in terms of the
charge {\em asymmetry} of the macro-ions, defined by their preference for a
non-zero effective charge, and their {\em donor/acceptor} propensity for
exchanging salt ions with the bathing solution. Asymmetric macro-ions tend to
increase the screening, while symmetric macro-ions can in some cases decrease
it. Macro-ions that are classified as donors display a rather regular behavior,
while those that behave as acceptors exhibit an anomalous non-monotonic Debye
length. The screening properties, in their turn, engender important
modifications to the disjoining pressure between two charged surfaces. Our
findings are in particular relevant for solutions of proteins, whose exposed
amino acids can undergo charge dissociation/association processes to/from the
bathing solution, and can be considered as a solution of charged regulated
macro-ions, as analyzed here.
