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Re: Feathers for S excretion



David Marjanovic wrote:

<<<And, I'll reiterate my first statement that kidneys make short work of
even massive loads of S.>

And I'll continue to doubt this. If organic sulfur compounds accumulate, they
must either sooner or later be metabolised, which produces H2S, or used
otherwise, such as in feathers. Of course kidneys deal well with massive
loads of urea and uric acid, but unlike for (likewise poisonous) NH3 there is
no such safe form of H2S. >>

You make a big  error if you assume organic sulfur leads to H2S. The body has
many pathways for sulfur. The main one is the cystathionine shunt, which
interconverts cysteine and methionine by swapping sulfur. Methionine is
oxidized through the sulfoxide to the sulfone, which I believe goes on to
yield sulfate. Sulfate can be concentrated 50 fold easily by the kidneys,
nearly matching their 100 fold ability for urea nitrogen. Sorry, but if you
add the major steroid clearing pathway of taurocholic esterification and bile
excretion, the out-shipping of sulfur is immense. There's just not likely to
be any significant need of an auxiliary "feather-shunt" pathway. If all of
the above isn't enough, there are well known medical conditions such as
homocystinuria, where genetic defects in metabolism lead to alternative
secretion mechanisms for sulfur, STILL not including any epidermal routes.
The kidney has an awesome capacity to adapt.

Thomas P. Hopp
Author of DINOSAUR WARS, a science fiction novel published by iUniverse
Now Humans are the Endangered Species!  http://members.aol.com/dinosaurwars