Effect of the treatment with thyroxin on certain enzymatic systems dependent on vitamin B6

Abstract

The well known antagonism between tyroxiu and vit. BG has prompted theí€ AA. to study theí€ effect of a treatment with tyroid hormon on some enzymes having pyridoxal-5-phosphate as cofactor. From a strictly biochemical point of view the antagonism tyroxin-B0 is not always easily demonstrable : after a treatment with tyroxiu theí€ liver glutamic-pyruvic transaminase is unchanged while theí€ glutamic-oxalacetic transaminase in liver heart is augmented. After the same treatment, brain glutamic decarboxylase and liver and brain 5-OH-tryptophan decarboxylase are unchanged, while liver eysteie acid decar­ boxylase is strongly impaired. Cysteic decarboxylase is only partially restored by «in vitro» addition of pyridoxal-5-phos- phate : thus both apoenzyme and coenzyme are affected by thyroxin treatment.

Coenzyme dephosphorylation, protein catabolism and adap- tativ phenomena are taken into account in order to explain the different behaviour of theí€ BG-dependeut enzymes.