Biology Reference
In-Depth Information
complex, probably composed of four identical subunits, i.e. a tetramer,
and that each subunit contained a single binding site for cGMP.
Strong evidence had also also been presented that only
background light activated the cyclase activity (Matthews
et al
.,
1988
; Pepe
et al
.,
1989
; Kawamura,
1993
). Hence, it was suggested
that signals from bleached photopigment may control the level of
cGMP in a reciprocal manner by their action on one or more of the
biochemical steps linking photon absorption to the hyperpolarizing
change of the plasma membrane potential without affecting the
cyclase activity (Stabell
et al
.,
1992
).
Light adaptation, on the other hand, appeared to be determined
by the balance between PDE hydrolysis and cyclase synthesis of
cGMP (Fain
et al
.,
1989
; Fain & Matthew,
1990
): an increase of illumi-
nation level would transiently lead to an imbalance in favour of the
PDE hydrolysis. The reduced level of cGMP would then in turn be
followed by an increase in cyclase activity, so that a new balance at a
higher level would be obtained.
Yet, important questions remained. Thus, for example, it had
long been known that calcium, in addition to its effect on guanylate
cyclase activity, also had significant effects on one or more of the other
biochemical steps of the excitatory cascade (Fain
et al
.,
1989
; Nicol
& Bownds,
1989
). A detailed description of present day knowledge of
the extremely complex processes and mechanisms of phototransduc-
tion has been given by Burns & Lamb (
2004
).
