The composition of isolated nuclei and cell preparations from tissues of calf, beef, horse, and fowl was studied with respect to the following components:

1. Liver and kidney arginase, catalase, and uricase; pancreatic lipase and amylase; cardiac muscle myoglobin; erythrocyte hemoglobin; intestinal alkaline phospharase. These are referred to as "special" components in view of their characteristically restricted distribution reflecting the differentiated nature of the tissues in question.

2. Esterase, ß-glucuronidase, alkaline and nucleotide phosphatases, adenosine deaminase, guanase, and nucleoside phosphorylase. These are enzymes of general distribution.

The differences in nuclear composition noted with respect to the "special" components, together with the broad variability in nuclear activity found for enzymes of general distribution, led to the conclusion that nuclei are differentiated structures.

The following distribution was observed:

1. "Special" components: Hemoglobin was found to be present in fowl and goose erythrocyte nuclei, but myoglobin was entirely absent from heart muscle nuclei; of the special enzymes listed, only catalase and arginase appeared to be concentrated in some of the nuclei. There was no significant nuclear concentration of lipase, amylase, uricase, or alkaline phosphatase. No simple relationship was found between the concentration of a special enzyme in a tissue and its activity in the corresponding nuclei. For example, arginase activity, which is high in mammalian liver and in fowl kidney, was found in liver, not kidney, nuclei. Similarly, catalase activity was demonstrated only in mammalian liver nuclei, although, in mammals, both liver and kidney are rich sources of this enzyme.

2. Enzymes of general distribution fell into three classes:

(a) Those present in low concentrations, if at all, in the nuclei—alkaline phosphatase, the nucleotide phosphatases) and ß-glucuronidase.

(b) Those present in nuclei in varying concentrations—esterase.

(c) Those present in high proportions in most nuclei—adenosine deaminase, nucleoside phosphorylase, and guanase. The exceptionally low nuclear activity of intestinal mucosa with respect to these enzymes was discussed in relation to physiological considerations.

The response of nuclei to changes in physiological state was demonstrated by experiments on starvation. The outstanding aspect of this response was a change in nuclear enzymatic activity opposing that observed in the cytoplasm.

A comparison of fetal and adult mucosa cells led to the following tentative interpretation of the observed intracellular enzyme distribution: In cells tending to moribundity, as in those subjected to starvation, relative nuclear enzymatic activity falls.

The occurrence of special enzymes in nuclei was considered in terms of differentiation, and the high nuclear concentration of the nucleoside-specific enzymes was interpreted in terms of general nuclear metabolic activity.

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