In
crude extracts of epidermal
papillomas induced by an initiation-promotion protocol,
ornithine decarboxylase (OrnDCase) activity was increased by the addition of
GTP to the
enzyme assay. No effect of
GTP on the
phorbol ester-induced
enzyme isolated from normal epidermis was observed. Kinetic analyses indicated that the major effect of the
nucleotide on the
tumor-derived
enzyme was to lower the apparent Km for L-
ornithine. When
papilloma OrnDCase was partially purified by gel-filtration chromatography, two forms of the
enzyme were resolved, only one of which was found in an epidermal extract from
phorbol 12-myristate 13-acetate-treated mice. The enzymatic properties of the two forms of
papilloma enzyme were compared. The higher molecular weight form (peak I) was activated by
GTP, while the lower molecular weight form (peak II) was not. As expected from the kinetic analyses of the crude
papilloma extracts, the apparent Km of peak I
enzyme for L-
ornithine was very high (1.25 mM) but was much lower in the presence of
GTP (0.02 mM). The two forms of
papilloma OrnDCase differed in their sensitivities to heat inactivation and the ability of
GTP to protect against heat inactivation. The K1/2 for activation of peak I OrnDCase by
GTP was 0.1 microM. The activation process was irreversible and did not require Mg2+. When several
nucleotides were tested for their ability to activate peak I OrnDCase, only
GTP,
dGTP, and the nonhydrolyzable derivative
GTP[gamma-S] were effective, while
GDP, GMP,
ATP, and
CTP were relatively ineffective. Our results demonstrated the existence of two forms of OrnDCase in epidermal
tumor extracts, of which one can be activated by
GTP and one cannot. The significance of these findings for the regulation of this
enzyme in normal and
tumor cells is discussed.