Enzyme circadian rhythms and conformational oscillators survey and prospects

Circadian and ultradian clock-controlled rhythms in unicellular microorganisms

The time structure of a biological system is at least as intricate as its spatial structure. Whereas we have detailed information about the latter, our understanding of the former is still rudimentary. As techniques for monitoring intracellular processes continuously in single cells become more refined, it becomes increasingly evident that periodic behaviour abounds in all time domains. Circadian timekeeping dominates in natural environments. Here the free-running period is about 24 h. Circadian rhythms in eukaryotes and prokaryotes allow predictive matching of intracellular states with environmental changes during the daily cycles. Unicellular organisms provide excellent systems for the study of these phenomena, which pervade all higher life forms. Intracellular timekeeping is essential. The presence of a temperature-compensated oscillator provides such a timer. The coupled outputs (epigenetic oscillations) of this ultradian clock constitute a special class of ultradian rhythm. These are undamped and endogenously driven by a device which shows biochemical properties characteristic of transcriptional and translational elements. Energy-yielding processes, protein turnover, motility and the timing of the cell-division cycle processes are all controlled by the ultradian clock. Different periods characterize different species, and this indicates a genetic determinant. Periods range from 30 min to 4 h. Mechanisms of clock control are being elucidated; it is becoming evident that many different control circuits can provide these functions.

Circadian variations of adenosine and of its metabolism. Could adenosine be a molecular oscillator for circadian rhythms?

The present review describes the biological implications of the periodic changes of adenosine concentrations in different tissues of the rat. Adenosine is a purine molecule that could have been formed in the prebiotic chemical evolution and has been preserved. The rhythmicity of this molecule, as well as its metabolism and even the presence of specific receptors, suggests a regulatory role in eukaryotic cells and in multicellular organisms. Adenosine may be considered a chemical messenger and its action could take place at the level of the same cell (autocrine), the same tissue (paracrine), or on separate organs (endocrine). Exploration of the circadian variations of adenosine was planned considering the liver as an important tissue for purine formation, the blood as a vehicle among tissues, and the brain as the possible acceptor for hepatic adenosine or its metabolites. The rats used in these studies were adapted to a dark-light cycle of 12 h with an unrestrained feeding and drinking schedule. The metabolic control of adenosine concentration in the different tissues studied through the 24-h cycle is related to the activity of adenosine-metabolizing enzyme: 5'-nucleotidase adenosine deaminase, adenosine kinase, and S-adenosylhomocysteine hydrolase. Some possibilities of the factors modulating the activity of these enzymes are commented upon. The multiphysiological action of adenosine could be mediated by several actions: (i) by interaction with extracellular and intracellular receptors and (ii) through its metabolism modulating the methylation pathway, possibly inducing physiological lipoperoxidation, or participating in the energetic homeostasis of the cell. The physiological meaning of the circadian variations of adenosine and its metabolism was focused on: maintenance of the energetic homeostasis of the tissues, modulation of membrane structure and function, regulation of fasting and feeding metabolic pattern, and its participation in the sleep-wake cycle. From these considerations, we suggest that adenosine could be a molecular oscillator involved in the circadian pattern of biological activity in the rat.

In vitro slow fluctuation in horseradish peroxidase activity

In vitro slow fluctuations in the level of horseradish peroxidase activity were observed in long-range experiments (72-144 h). Besides random fluctuations, regular slow oscillatory patterns with period lengths ranging from 10.0 to 39.0 h were detected by statistical analysis. The possibility that these oscillations in enzyme activity could have reflected changes in the physical environment of the experimental setup has been thoroughly examined and ruled out. Periodic exposition of the enzyme solution, otherwise kept in darkness, to blue light illumination was shown to influence the period of the oscillations. The changes in enzyme activity were correlated with a modification of the Michaelis constant estimated using guaiacol as substrate. This result was confirmed by the action of chemical modifiers of the enzyme, such as ferulic acid and rutin. It is thought that the observed oscillations in horseradish peroxidase activity are due to spontaneous and specific changes in the tridimensional structure of the enzyme in the thermic reservoir.