Abstract
It was shown [Goldbeter & Koshland (1981), Proc. Natl. Acad. Sci. USA, 78, 6840–6844] that amplified sensitivity may arise in reversible covalent modification systems, when the converter enzymes operate in their zero-order region. we show that “zero-order ultrasensitivity” may also occur in simple substrate cycles. The experimental study deals with the Formate/Lactic dehydrogenases model cycle, interconverting the reduced and oxidized forms of NAD. For NAD(H) concentrations high enough (with respect to the enzyme KM's), abrupt changes in the steady-state substrate concentrations may result from small variations in the ratio of maximal enzyme activities. The amplification factors are measured. Implications in metabolic regulation are also taken up.
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