Add time:07/20/2019         Source:sciencedirect.com

The effects exerted by carbamoyl phosphate (CP) and cyanate (KCNO) on rat liver l-threonine deaminase have been studied. The two compounds showed the same effects, inhibiting through a competitive mechanism both the holoenzyme and the dialyzed enzyme; inhibition was more evident for the latter. Ki values, both for l-threonine and pyridoxal 5′-phosphate (PLP), were lower for the apoenzyme and the inhibitors also affected the Km of the apoenzyme for PLP. The effects of CP and KCNO are mainly due to an interface in the association reaction apoenzyme + PLP holoenzyme This was clearly demonstrated by the fact that, when PLP was incubated with CP or KCNO, it failed to enhance the activity of the holoenzyme nor did it reactivate the resolved apoenzyme. Such interference of CP and KCNO in the l-threonine deaminase activity was mainly due to a specific mechanism, the formation of a new derivative of PLP. The reaction of PLP with either CP or KCNO occurred readily, at low concentrations, under physiological conditions. The new compound was identified as 3,4-dihydro-2H-pyridol[3,4-e]1,3-oxazin-2-one derivative by ultraviolet-visible spectra, elemental analysis, infrared, NMR and MS spectra. In this paper we formulate the hypothesis that this compound is involved in the regulation of the CP and PLP intracellular content and in the control of PLP dependent enzymes.

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