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David Walker david at alegba.demon.co.uk
Mon Apr 22 11:52:02 EST 1996


To: photosyn at net.bio.net

Date: Fri, 05 Apr 1996 11:11:49 -0600

From: feild taylor s <feild at ux5.cso.uiuc.edu>

Subject: Km's for Mehler reaction

I am curious to know if any one could direct me to sources or provide me
with Km values oxygen utilization via the Mehler reaction and
mitochondrial respiration. Any units (micromoles, microbars, %) would be
great. I found one estimate for the Mehler reaction in chloroplasts
around 1% O2 published by Schreiber. Is there any variabilty tied to
that number?

Taylor Feild
feild at students.uiuc.edu

During some ancient work we inhibited catalase activity with azide in order
to observe the time course of the Mehler reaction. On illumination, O2
uptake continued at an unchanged rate from the 'air line' to the 'nitrogen
line' implying a very high affinity for O2 . If, at any time during this
period of O2 uptake, illumination was stopped and excess catalase added,
the O2 trace returned to the airline in accord with the stoichiometry of
the Mehler reaction. However, if illumination was allowed to continue until
the O2 trace had reached the nitrogen line and then to run along it (i.e.
illumination now in the absence of O2 but in the presence of hydrogen
peroxide generated by the Mehler reaction) the cessation of  illumination
and the addition of xs catalase yielded smaller and smaller amounts of O2.
In other words there was a mechanism, for which I have never yet seen a
fully convincing explanation, which brought about consumption of H2 O2  in
the light but only in the absence of O2.
Some of this was published in:-

WALKER DA, LJ LUDWIG, DG WHITEHOUSE 1970  Oxygen evolution in the dark
following illumination of chloroplasts in the presence of added manganese.
FEBS Letters 6: 281-284

WHITEHOUSE DG, LJ LUDWIG, DA WALKER 1971  Participation of the Mehler
reaction and catalase in the oxygen exchange of chloroplast preparations. J
Exp Botany 22: 772-791

A much more recent paper is also relevant:-

WALKER DA, BAILEY KJ 1992 Changes in fluorescence quenching brought about
by feeding dithriothreitol to leaves.  Plant Physiology 99: 124-129


David Walker

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