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Re: Feathers for S excretion



Sorry for the long post... I really think it's worth it :-)

> Interesting Reichholf translations. However, I think he needed some more
> basic biochemistry before he got started.

He makes the impression as knowing about biochemistry; maybe we should get
himself into the discussion -- his 1998 paper gives his address:
Prof. Dr. Josef H. Reichholf, Zoologische Staatssammlung, Abteilung
Wirbeltiere, Münchhausenstr. 21, D-81247 München.

>    H2S? I wasn't aware the body produced ANY H2S in metabolizing proteins.
> That's really news to me (another way of saying I'm deeply skeptical that
it
> is a significant factor).

Yet H2S seems to be produced, just in minute quantities _normally_.

> If insects contain only 3% S, then
> they are not a particularly burdensome source of S after all. Reichholf's
> basic datum seems weak.

I can't comment this -- see above.

>    The notion that feathers have a "special" sort of sulfur requirement is
> flawed. The S, primarily as cystine, participates in crosslinking the
> proteins of feathers to give them strength. Trouble is, ALL skin
structures
> have this same crosslinking. Feathers, hairs, scales and even just plain
skin
> ALL have S-linked crossbridges between their proteins. So there is no real
> evolutionary novelty in the S content of feathers.

Trouble? Feathers just seem -- I don't have primary literature on this -- to
contain MORE cystine crosslinks than anything else. Reichholf seems to claim
this in his 1998 paper, though his sentences are rather ambiguous:

"Without exception feathers consist of [...] keratin; a protein that, by
sulfur bridges (disulfide bridges [cystine crosslinks]) between the
microfibrils of the helical basic structure, reaches a completely
exceptional strength and elasticity. With respect to lightweightedness and
elasticity keratin surpasses probably all metals and alloys and rivals the
best of modern plastics. The specific form of feather keratin is in this
respect clearly distinct from the simpler forms which occur in the scales of
reptiles or also in the hair of mammals or on the skin surface as horn
layer.
        The production of feathers thus requires certain amounts of protein,
more precisely: amino acids, and their production [probably of feathers and
not amino acids] costs the bird body not only relatively 'precious'
substances, but also energy."

But:

>    As far as an insect diet, 3% sulfur is not a great amount, in fact, it
is
> just slightly above typical levels among all known proteins. Don't forget,
S
> is a heavier atom than, say, carbon or nitrogen, so it's weight-percent is
> exaggerated relative to its mole-percent.

Though it is probable that the 3 % are weight-%, Reichholf never says what
kind of % they are supposed to be... But in his 1998 paper, he explicitely
writes:

"But keratin chains that are particularly rich in the amino acid cysteine
are connected with one another by disulfide bridges to cystine [the cysteine
dimer]. Cystine can, as research on songbird plumage has shown, make up for
11 mol% of all amino acids. The sulfur content is over 3 percent! Thus
feathers are particularly sulfur-rich (4). Where does this sulfur come from?
[...] [Diet.] Now the question is, which way would the sulfur that would not
be needed by the rest of the body if no feathers would have to be produced
have to take?" H2S production is too poisonous, as I quoted yesterday,
sulfate production may be cheap enough, but feather production would use up
lots of excess sulfur-containing amino acids, which in turn provides an
explanation for moulting (the only one I know so far).

And even if not -- feathers are simply a LOT of keratin and therefore
contain a lot of sulfur anyway!

Probably the hard protein matrices in which chitin is embedded in insects
also has more cystine than more ordinary epithelia. BTW, the 3 % are the
sulfur content of feathers, not that of insects, for which he gives no
number.

>    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.