Comparative Studies in vivo and in vitro of Rat‐Liver Enzymes

The stability of a number of enzymes which have a very short half‐life as assessed by measurements in vivo , namely ornithine decarboxylase, serine dehydratase, tyrosine aminotransferase and 5‐aminolevulinate synthase, and the stability of enzymes with a moderate half‐life i.e. glutamate dehydrogenase, alanine aminotransferase and lactate dehydrogenase, were studied in whole liver and in tissue fractions thereof. While in general the measurements in vitro are in agreement with the data obtained in vivo , indicating the power of the simple technique herein described, there was enough variation in stability to indicate that in addition to their intrinsic basic stability due to the particular genetically determined structure of the protein, environmental factors must play an important role in the half‐life of these enzymes. For example, tyrosine aminotransferase and particularly serine dehydratase are less stable in the tissue than in the homogenate and supernatant fractions and consequently are less stable in vitro than in vivo. Lactate dehydrogenase and alanine amino‐transferase are stable for as long as 18 h in the homogenate and supernatant; however alanine aminotransferase activity in tissues decreases after 6 h incubation. The half‐life of inactivation in the homogenate for glutamate dehydrogenase is 3–4 h. Interestingly, the activity in the tissue increases up to 6 h, while with longer periods of incubation the bulk of the enzyme leaked out into the medium and remained constant for 24 h. Experiments with “washed‐residue” fractions show that the enzyme is inactivated by cytosol components. No clear difference between inactivation in vitro before and after starvation, following a high‐protein diet or after injection of inhibitors was found for any of the enzymes studied.