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Re: Feathers for S excretion (very long, as usual)
----- Original Message -----
Sent: Tuesday, March 27, 2001 4:53 AM
Subject: 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,
> 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
> no such safe form of H2S. >>
> You make a big error if you assume organic sulfur leads to H2S.
H2S does occur as an end product of sulfur metabolism.
> The body has
> many pathways for sulfur. The main one is the cystathionine
What is cystathione?
> 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 is indeed produced, and Reichholf knows this. Directly to what I
quoted last time, he writes in his 1998 paper:
"In 'normal' metabolism sulfur would either accumulate as hydrogen sulfide
(H2S) [...] or would have to be built into sulfate (SO4 2-). Hydrogen
sulfide is quite poisonous, especially at the over 40 °C high inner body
temperatures of the bird [sic]. Thus the question remains of what is
"cheaper", i. e. energetically and metabolism physiologically less
demanding, sulfate production of excretion through the skin in the form of
feather keratin? The isolated case [my dictionary is really great] won't be
able to clarify this question because bird lifestyles are too diverse for
that. But it [the question] can be reformulated with respect to the
principle: How would this be if protein, rich in sulfur-containing amino
acids, is not rare in the diet, but in excess? Then using a large part of it
for feather production would constitute a second disposal pathway and
moulting would get the aspect of excretion of excesses from the body. In
this case it would not be surprising at all that "good" feathers are
moulted, and the recent observations on the deposition of harmful substances
in bird plumage could be looked at in new light (6). What would such a view
require in advance? Firstly surely that the diet would have to include, at
least part of the time, a protein content exceeding the needs, protein that
would additionally have to be rich in sulfur-containing amino acids. But a
reduction of protein intake [eating less] to the needed level would have to
counteract another need that would have to be satisfied and could not be
measurably lowered. [...] [Means, eating less protein also means eating less
fat and carbohydrates, but these two are needed in large amounts, so eating
less protein is impossible.]" Burning protein yields, in contrast to fat and
carbohydrates, end products that must be actively excreted. I don't really
understand what the next few sentences imply: "In birds this is (the badly
water-soluble) ureic acid. Its excretion saves water, but all the more it
concentrates the product of kidney excretion, especially when metabolism
runs at full gear. The problematics of sulfur excretion results from this."
Probably this alludes to the fact that many sulfates (sodium sulfate...) are
well water-soluble and thus can't be excreted without water, so HP Tom
> Sulfate can be concentrated 50 fold easily by the kidneys,
> nearly matching their 100 fold ability for urea nitrogen.
may well apply to mammal kidneys, but probably not to bird kidneys.
ref. (6) = M. Niecke & A. Krüger: Schwermetalle in Vogelfedern --
Systematische Untersuchung von externer Deposition und Einlagerung in die
Feder. Beiträge zur Gefiederkunde und Morphologie der Vögel 4 (1997), S.
1 -- 20 ( = Heavy metals in bird feathers -- systematical investigation of
external deposition and deposition into the feather [sic]. "Contributions to
plumage-ology and morphology of birds", which must be a very obscure
Reichholf goes on:
"[Migratory] birds need extraordinarily much energy [...] The main fuel
[during migrations] is fat [...] Birds that migrate far must therefore have
become "migration-fat" [before they migrate] -- and this they become in
connection with moulting. It is not exactly known what happens here but
there is much evidence that the storing of fat for migration brings about an
excess of protein that must be gotten rid of.
A similar situation arises when female birds produce in short time their
relatively very large eggs (not seldom several to many in a clutch!) and
have to provide it with the contents necessary for the development of the
embryo. Disequilibria in substance intake and metabolism are impossible to
avoid in this short-time situation. They also lead to considerable
differences in the nutrition-physiological demands on females and males. The
latter "work off" [...] their protein excesses in the form of the
development of display plumage and their energy excess in demanding
courtship behaviors that apparently correlate to the size of the clutches of
the females. [...]
_A new model of feather evolution_
The previous models of feather origins are oriented at interactions with the
environment. They deal with the 'outside'. Scale enlargements that became
feathers in the course of millions of years could become this because the
environment 'rewarded' minimal improvements with selection advantages. But
how many lizards with weakly enlarged scales would have had to fall down and
break their necks to give the bearers of the first, weak scale enlargements
the advantages? Or which degree of of fraying scale enlargements does a
measurable improvement of warmth isolation [...] require? [I can't imagine
the insulatory advantage of a half-protofeather or a spine either.] Models
that are oriented to 'adaptations' [why the quotation marks here?] to the
environment have to fight with this core problem nearly without exception.
It looks entirely different when the 'beginning' comes 'from inside'. If
reptiles from which birds evolved had had an excess of protein with
sulfur-containing amino acids in their diet for a very long time, we don't
need to assume any outside-directed selection advantages at all for scale
enlargements and fine-structural changes of the keratin that result from the
enhanced use of sulfur-containing amino acids. They are simply the result of
inner excesses and their disposal (3)."
(3) = J. H. Reichholf: Die Feder, die Mauser und der Ursprung der Vögel.
Archaeopteryx 14 (1996), S. 27 -- 38. This must probably have an English
abstract, and HP Darren Naish may be able to find this article
(Archaeopteryx seems to be unavailable over here, even though, say,
Vertebrata PalAsiatica is not).
> Sorry, but if you
> add the major steroid clearing pathway of taurocholic esterification and
> excretion, the out-shipping of sulfur is immense.
Oh! Are there steroids and bile pigments that contain sulfur? I don't know
> 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.
What exactly is homocystinuria?
> The kidney has an awesome capacity to adapt.
But, apparently, no capacity to excrete sulfate without water.