Another finding of this study is the endogenous regulation of ACE activity by inhibitors. The first results on potential endogenous inhibitors of ACE were reported as early as 1979 [ 35]. Later human results also suggested the existence of endogenous ACE inhibitors in the heart [ 36] as well as in the serum by identifying C-type natriuretic peptide [ 37]. Moreover, it was also shown that dilution can be a valuable tool to investigate the endogenous inhibition of ACE [ 38], suggesting that ACE is generally inhibited in rat tissues. Our previous reports on the endogenous inhibition of circulating ACE activity [ 17] by serum albumin [ 18] were confirmed in the present study. Applying the same technique, we observed a significantly higher endogenous inhibition (approximately 70%) in lung tissue than in blood. These ACE inhibitory levels were comparable in patients with and without ACE inhibitory medications, suggesting a negligible effect of the drug on tissue ACE activities. The concentration of human serum albumin is too low in the lung tissue samples to provide significant ACE inhibition [ 18], and thus, this implicates an alternative mechanism for ACE inhibition in the present study. These findings were in accordance to that found in the rat, suggesting at least 85% endogenous ACE inhibition in the lung [ 38]. Further studies are required to identify the molecular nature of the endogenous ACE inhibitor in human lung tissue. Using this genotype-dependent expression pattern as a tracer, we tested if the primary source of circulating ACE is the lung in humans. To do so, we tested ACE levels in serum and lung samples of the same patients in parallel, using techniques developed in our laboratory in the past years [ 17, 18, 20, 27, 28]. Patients with the DD genotype had significantly higher circulating ACE concentrations and activities than patients with the ACE II genotype, while patients with the ACE ID genotype showed intermediate values confirming earlier reports. However, we did not find any correlation of lung tissue ACE expression or activity with the ACE I/D genotype. This finding suggests that ACE expression in the lungs is independent of ACE I/D genotype, and consequently, the genotype-dependent serum ACE secretion must have an alternative source of ACE. Danilov, S.M.; Tovsky, S.I.; Schwartz, D.E.; Dull, R. ACE Phenotyping as a guide toward personalized therapy with ACE inhibitors. J. Cardiovasc. Pharmacol. Ther. 2017, 22, 374–386. [ Google Scholar] [ CrossRef] Our data suggest that the source of circulating ACE is independent of lung capillaries. In line with that, the human heart was identified as an alternative source for circulating ACE. Additional ACE expressing and secreting cells can also be found in the apical surface of epithelial cells in the proximal tubule of kidney, the mucosa of small intestine, the syncytial trophoblast of placenta and the choroid plexus, in addition to various regions within the central nervous system [ 24]. Moreover, ACE was also found to be expressed by macrophages [ 33]. While the role of these potential ACE sources in the circulating ACE levels is unknown, it is well established that circulating ACE level increases in sarcoidosis [ 34]. We also confirmed elevated circulating ACE levels in patients with sarcoidosis and proposed that it can be used as a biomarker for sarcoidosis [ 27, 28]. Using a similar approach to ours, an independent study reported genotype-dependent ACE expression in the human heart [ 15] in full accordance with our findings in the present study, suggestive of a relationship between serum and cardiac ACE activities.

König, I.R.; Fuchs, O.; Hansen, G.; von Mutius, E.; Kopp, M.V. What is precision medicine? Eur. Respir. J. 2017, 50, 1700391. [ Google Scholar] [ CrossRef] Metzger, R.; Franke, F.; Bohle, R.-M.; Alhenc-Gelas, F.; Danilov, S.M. Heterogeneous distribution of Angiotensin I-converting enzyme (CD143) in the human and rat vascular systems: Vessels, organs and species specificity. Microvasc. Res. 2011, 82, 206–215. [ Google Scholar] [ CrossRef] [ PubMed]Ching, S.F.; Hayes, L.W.; Slakey, L.L. Angiotensin-converting enzyme in cultured endothelial cells. Synthesis, degradation, and transfer to culture medium. Arteriosclerosis 1983, 3, 581–588. [ Google Scholar] [ CrossRef] [ PubMed] According to a widely accepted consensus, ACE is expressed primarily by endothelial cells, particularly those of the lung [ 14], and subsequently released into the circulation. However, the human heart also expresses ACE [ 15], suggesting that the lung is probably not the only organ contributing to circulating ACE in humans. Moreover, levels of ACE expressions in kidneys and in small intestines were also found to be comparable to those in the lung [ 16].

Kost, O.A.; Grinshtein, S.V.; Nikolskaya, I.; Shevchenko, A.; Binevski, P.V. Purification of soluble and membrane forms of somatic angiotensin-converting enzyme by cascade affinity chromatography. Biochemistry 1997, 62, 321–328. [ Google Scholar] Wei, L.; Gelas, F.A.; Soubrier, F.; Michaud, A.; Corvol, P.; Clauser, E. Expression and characterization of recombinant human angiotensin I-converting enzyme. Evidence for a C-terminal transmembrane anchor and for a proteolytic processing of the secreted recombinant and plasma enzymes. J. Biol. Chem. 1991, 266, 5540–5546. [ Google Scholar] [ CrossRef] In Tales of the TARDIS, Ace reunited with an aged Seventh Doctor and discussed their adventures. They also discussed the meeting with Fenric and both apologised to each other falling out. Iwatani, S.; Yamana, K.; Nakamura, H.; Nishida, K.; Morisawa, T.; Mizobuchi, M. A Novel Method for Measuring Serum Unbound Bilirubin Levels Using Glucose Oxidase-Peroxidase and Bilirubin-Inducible Fluorescent Protein (UnaG): No Influence of Direct Bilirubin. Int. J. Mol. Sci. 2020, 21, 6778. [ Google Scholar] [ CrossRef] Ahlfors, C.E.; Vreman, H.J.; Wong, R.J.; Bender, G.J.; Oh, W.; Morris, B.H.; Stevenson, D.K.; Subcommittee, T.P. Effects of sample dilution, peroxidase concentration, and chloride ion on the measurement of unbound bilirubin in premature newborns. Clin. Biochem. 2007, 40, 261–267. [ Google Scholar] [ CrossRef]Under the Doctor's tutelage, Ace fights the Daleks in Remembrance of the Daleks, the Cybermen in Silver Nemesis, encounters the all-powerful Gods of Ragnarok in The Greatest Show in the Galaxy, the sadistic torturer Kandy Man in The Happiness Patrol, and many other dangers. She also faces the ghosts of her own past in Ghost Light and The Curse of Fenric, coming to terms with them and, ironically, creating them in the latter case thanks to the paradoxes of time travel. Over time, she starts to mature into a confident young woman, and her brash exterior ceases to be a front. Kryukova, O.V.; Tikhomirova, V.E.; Golukhova, E.Z.; Evdokimov, V.V.; Kalantarov, G.F.; Trakht, I.N.; Schwartz, D.E.; Dull, R.O.; Gusakov, A.V.; Uporov, I.V.; et al. Tissue Specificity of human angiotensin I-converting enzyme. PLoS ONE 2015, 10, e0143455. [ Google Scholar] [ CrossRef] [ PubMed] In 2018, Ace was confirmed to appear in Big Finish Production's audio drama based on Doctor Who spin-off Class, with Aldred reprising her role alongside the show's main cast. [13] The story released that August.