Toxicology 133 (1999) 139 – 145
Inhibition of human erythrocyte Ca
2 +
-ATPase by Zn
2 +
Christer Hogstrand
a,
*, Pieter M. Verbost
b
, Sjoerd E. Wendelaar Bonga
b
a
T.H. Morgan School of Biological Sciences, 101 Morgan Building, Uniersity of Kentucky, Lexington, KY 40506 -0225, USA
b
Department of Animal Physiology, Uniersity of Nijmegen, Toernooield 1, 6525 ED Nijmegen, The Netherlands
Received 23 September 1998; accepted 4 February 1999
Abstract
Recent investigations suggest that Ca
2 +
-ATPase from fish gills is very sensitive to Zn
2 +
(Hogstrand et al., 1996.
Am. J. Physiol. 270, R1141–R1147). The effect of free Zn
2 +
ion on the human erythrocyte plasma membrane
Ca
2 +
-ATPase was investigated to explore the possible extension of this finding to humans. Membrane vesicles were
prepared and the Ca
2 +
-ATPase activity was measured as Ca
2 +
-stimulated ATP hydrolysis and as ATP-dependent
Ca
2 +
transport. The Zn
2 +
ion inhibited the erythrocyte Ca
2 +
-ATPase by reducing V
max
and increasing the K
0.5
.
While in the Ca
2 +
transport assay only the V
max
was affected at lower Zn
2 +
concentrations (50 – 100 pM), reduction
of V
max
was always accompanied by an affinity decrease in the ATP hydrolysis assay. The Ca
2 +
-ATPase was found
to be inhibited by Zn
2 +
at extremely low concentrations. The IC
10
and IC
50
for Zn
2 +
, at a Ca
2 +
concentration of
1.0 M, were estimated at 4 and 80 pM, respectively. Although the Ca
2 +
-ATPase might be more sensitive in vitro
than in vivo conditions, the results suggest that physiological concentrations of Zn
2 +
may reduce the activity of the
erythrocyte Ca
2 +
-ATPase. Furthermore, disturbance of Ca homeostasis may be a mechanism causing Zn toxicity
during exposure. © 1999 Elsevier Science Ireland Ltd. All rights reserved.
Keywords: Zinc; Calcium; Transport; Red blood cells; Inhibition; Membrane
1. Introduction
In a recent study, ATP-dependent Ca
2 +
trans-
port in plasma membranes of rainbow trout gills
was found to be sensitive to Zn
2 +
(Hogstrand et
al., 1996). Concentrations down to 100 pM of free
Zn
2 +
significantly reduced the activity of the ba-
solateral high-affinity Ca
2 +
-ATPase by mixed in-
hibition and there was possible, but non-
significant, inhibition even at 50 pM of Zn
2+
. It
was earlier shown that interference with transep-
ithelial Ca
2 +
transport is a likely mechanism of
Zn toxicity to fish (Spry and Wood, 1985;
Hogstrand et al., 1994). Thus, this effect can now
be explained at the enzymatic level. From some
* Corresponding author.
E-mail address: hogstra@pop.uky.edu (C. Hogstrand)
0300-483X/99/$ - see front matter © 1999 Elsevier Science Ireland Ltd. All rights reserved.
PII:S0300-483X(99)00020-7