The liver’s carbon fixation
The possibility that insects can harness solar energy (SN: 1/15/11, p. 8) is
no less fascinating than the ability of
the mammalian liver to do the light-independent part of photosynthesis:
carbon fixation. When concentrations
of the amino acid methionine rise after
a high-protein meal, the liver shifts
gears to get rid of the excess via activation of a specific transmethylation
pathway requiring the amino acid glycine as a methyl acceptor. This also sets
in motion what I call the “glycine generator” — a short cycle involving two
reversible folate-requiring enzymes
cranking out two moles of glycine for
each mole of serine, ammonia and
carbon dioxide. Copying this aspect
of nature on an industrial scale might
enable the recycling of substantial
amounts of carbon dioxide.
Joel Brind, New York, N. Y.
Brind is a professor of biology at Baruch
College, the City University of New York.
Light on genetic dark matter
“Genetic dark matter” (SN: 12/18/10,
p. 18) might be hiding in plain sight. For
over two decades, abundant variation in
the number of tandemly repeated units
in microsatellite and minisatellite DNA
has been used for genetic fingerprinting.
For years, this variation has been widely
regarded as functionally meaningless.
For much of that time, several biologists
(including myself ) have hypothesized
that such repeat-number variation
might help account for heritable variation in certain traits.
Most such repeats are indeed found
in genomic regions that lack known
function. But among the hundreds
of thousands of repeats scattered
throughout the human genome are
many that are closely associated with
genes. Lots of genes, perhaps most,
include at least one variable tandem
repeat sequence at sites where the
number of repeat units can influence
gene function. Nevertheless, most
attention (and most research investment) remains focused on single
nucleotide polymorphisms (SNPs) for
the very simple reason that collecting
vast data sets on SNPs has become
cheap and easy. Although surveying
repeat number variation and linking this
variation with phenotypic differences
are technically challenging, biologists
should not lose sight of this very visible
source of heritable variation.
Science News has covered some of
the relevant studies in past articles
(SN: 12/18/04, p. 387; SN: 1/31/09,
p. 26). The subject of repeat number
variation might be suitable for an article
that could shed light on many interrelated topics — triplet repeat diseases,
evolutionary facilitation, molecular
genetics, genomic diversity, etc.
David G. King, Carbondale, Ill.
King is an associate professor of zoology
at Southern Illinois University.
Heartburn drugs’ pros and cons
Nathan Seppa’s article (“It’s enough
to give you heartburn,” SN: 12/4/10,
p. 30) edges toward tabloid in tone.
Proton pump inhibitors and the like
have saved many lives and reduced
nearly to vanishing the need for peptic-ulcer surgery, and, as Seppa points
out, are important in the control of
common and dangerous gastroesopha-geal reflux disease. PPIs have proved
remarkably well tolerated in general.
Obviously, like all drugs, they should be
That said, in addition to the unusual
untoward effects of PPIs listed in the
article, two more might be mentioned.
Because hydrochloric acid is a first-line defender against gastrointestinal
infection, the risks of other gastrointestinal infections besides C. difficile
are likely increased. PPIs may also
cause tubulointerstitial nephritis.
Harvey E. Finkel, Brookline, Mass.
Finkel is a physician.
Showing that overuse of drugs can be
injurious to your health is not only
important from the standpoint of an
individual’s risk, but also for the over-
whelming bill for these drugs, paid
sometimes by the individual and at
other times by the insurer. There is
always a risk-benefit ratio for drugs.
Added to the equation should be the
cost of profligate use of expensive drugs.
Nelson Marans, Silver Spring, Md.
One cat not lapping
One of my cats does not lap. She puts
her paw in the water dish and then licks
her paw. I have left your December 4
issue (“Cats lap liquids with a flick of the
tongue and fluid dynamics,” SN: 12/4/10,
p. 5) open to the pictures of the cat
drinking, so perhaps she will get the idea.
Emily Johnston, Westminster, Md.
In “Snails shed shells in one fell swoop”
(SN: 11/6/10, p. 9), you describe an
experiment in which baby snails
exposed to the metal platinum develop
without external shells. This is hypothesized as a rapid evolutionary mechanism that could explain such events as
the transition from snails to slugs.
However, one must inquire whether
the creatures’ DNA was modified.
If these newly shell-less snails were
allowed to mature and procreate,
would their progeny (in the absence
of platinum) still develop shells? If so,
then what we are observing is more
akin to a birth defect, not evolution.
Irwin F. Kraus, Attleboro, Mass.
The scientists didn’t demonstrate that
the changes in snail body plan after
exposure to platinum were heritable,
and we don’t know if the next generation
would have had shells or not. What the
study did show is that a drastic change
in body plan doesn’t necessarily result
from slow, incremental changes — big
shifts in development, however induced,
can happen quickly. The work suggests
that it’s possible that some ancient snail
could have acquired genetic mutations
similar to those induced by the platinum
that may have had a large effect on body
plan and perhaps on snail and slug evolution. —Rachel Ehrenberg
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