3. Summary and conclusions

Energy and protein (amino acid) metabolism interact at various levels of biological complexity, as indicated above. It is not surprising, therefore, that changes in energy intake will give rise to a complex pattern of responses in amino acid and protein metabolism, depending in part upon overall nutritional background and host conditions involved. A number of the molecular and cellular processes that are involved in the interaction were summarized, and while this new knowledge has not yet helped us to better quantify the in vivo interrelationships between energy and protein metabolism, it has expanded our appreciation of the functional significance of dietary energy and protein interactions. It may also help stimulate, here at this workshop or when we return to our laboratories, new thinking about how to define and understand the mechanisms involved.

Molecular and cellular studies have shown, for example, that redox potential can affect the overall thiol-disulfide status of the cell, which in turn can influence the redox state of individual proteins (CAPPEL and GILBERT, 1988; ZIEGLER, 1985) and so determine the activity of metabolic processes. A recent example of this, from ABATE et al. (1990), concerns the DNA binding of the Fos-Jun heterodimers, which functions as an intermediary transcriptional regulator in signal transduction. This is modulated by reduction-oxidation (redox) of a single conserved cysteine residue in the DNA-binding domains of the two proteins, so implicating a possible redox mechanism in the regulation of transcriptional activity mediated by AP-1 binding factors. It appears that not only are ATP formation and availability significant factors in relation to the dynamic state of protein (and energy) metabolism, but also the pattern of fuel utilization (YOUNG et al., 1992b), through an influence on the redox state of the cell could be quite important in determining the quantitative relationships between protein turnover and body energy metabolism. As ZIEGLER (1985) points out, there is not yet sufficient evidence for a major role of changes in the redox state of cell proteins that modulate metabolic pathways in response to physiological stimuli. However this is a hypothesis that would be well worth exploring.

Similarly, it is to be expected that the level, and possibly source, of protein intake and status of protein nutriture would influence energy metabolism (e g., SAMONDS and HEGSTED, 1978; COYER et al., 1987; AUSMAN et al., 1989), mediated, in part, via effects on these various processes and promoted by alterations in the functioning and activity of the endocrine system (e.g., MILLWARD, 1990; YOUNG and MARCHINI, 1990; LONG and LOWRY, 1990). This emphasizes why it is worthwhile for us to explore, in further detail during this workshop, the metabolic interactions between energy and nitrogen and their nutritional implications. It is also still intriguing to clarify further the molecular and cellular features of the pathophysiology of the various forms of protein-energy malnutrition, with the extremes being marasmus and kwashiorkor. In doing so it is hoped that this basic knowledge would encourage development of more effective approaches for preventing significant tissue and organ protein depletion under conditions of stress, and for replenishing tissue and organ proteins in affected individuals and populations.