Guanine salvage by organ cultures of mouse tooth germs.

The effect of mycophenolic acid (MPA) which inhibits the biosynthesis of guanosine monophosphate (GMP) in organ cultures of mouse tooth germs can be partially counteracted by adding guanine to the MPA cultures. This may be due to salvaging guanine by the enzyme hypoxanthine guanine phosphoribosyl transferase (HGPRT), or to competition for a common membrane carrier involved in mediated transport of both guanine and hypoxanthine in normal biosynthesis and also of MPA. Experiments were carried out to compare the effect of either hypoxanthine or guanine on the MPA-caused inhibition. While addition of guanine to the MPA cultures (MPAG) supports growth equal to controls and development of dental-enamel junction (DEJ) to a level intermediate between control and MPA the addition of hypoxanthine (MPAHX) supports growth and DEJ development not better than MPA. This indicates that guanine is salvaged by HGPRT to GMP while hypoxanthine, salvaged to inosinic acid (inosinic monophosphate, IMP) is ineffective because the MPA inhibition is on the pathway from IMP to GMP.

Tooth germ epithelial-mesenchymal interactions in tissue culture.

The usefulness of the tooth germ in culture arises from the fact that it exemplifies those fundamental attributes of development, cell proliferation, cytodifferentiation, and morphogenesis, which we expect to find in the development of any metazoan organism. In culture, as in the organism, such development takes place in 3 dimensions. This study was undertaken to determine if it is possible to uncouple, by using 2 dimensions, cytodifferentiation from morphogenesis. Under the conditions used, cytodifferentiation in culture was not apparent (at the light microscope level). However, the following interesting observations were made: Cell populations arising from the same types of explants (enamel organ/enamel organ or dental papilla/dental papilla) readily flow together. Cell populations arising from dissimilar types of explants (enamel organ/dental papilla) form sharp boundaries at their interfaces. Additionally, cell populations arising from intact tooth germs differ from those arising from either enamel organs or dental papillae.

In vitro processing of the adenosine analog formycin A to the mono-, di-, and triphosphate by a soluble multienzyme system from mouse liver.

The adenosine kinase activity present in a soluble preparation from rat liver was investigated using formycin A (FoA), a fluorescent analog of adenosine as the phosphoryl acceptor and ATP as the donor. Reversed-phase high-performance liquid chromatography (h.p.l.c.) was used to separate substrate from product, and the progress of the phosphorylation reaction was followed by monitoring fluorometrically the amount of formycin 5'-monophosphate (FoMP), and the AMP analog, that was formed. The results showed that while FoMP was formed during the reaction indicating that an adenosine kinase activity was present, both formycin 5'-di- and triphosphate (FoDP and FoTP respectively), the corresponding analogs of ADP and ATP, were also formed, suggesting than an adenylate kinase activity was present. This result was confirmed with FoMP as the substrate and showing the formation of FoDP and FoTP. Other experiments carried out with FoMP as the substrate revealed the formation of FoA. Taken together, these results indicated that a 5'-nucleotidase activity as well as an adenylate kinase was present. Using this analog and h.p.l.c., it has been possible to demonstrate for the first time in an in vitro system the complete salvage of a nucleoside to the triphosphate level.