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Are there lactose sources other than dairy products in nature? I tried to search for it but I did not find anything. The question arose because I was wondering whether lactose-utilising enzymes in bacteria evolved only after the appearance of mammals.

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  • $\begingroup$ Some seem to argue that lactose can be found in some vegetables and fruits too. A common reference to the sources I could find is a paper by Toba et al. (1991) titled Is lactose really present in plants? Unfortunately, the paper is behind a paywall for me so I can't tell whether the answer they give to this question is "Yes" or "No"! $\endgroup$ – Remi.b Apr 27 '18 at 22:25
  • $\begingroup$ According to Toba et al. per @Remi.b the answer is no. Reinvestigation of reports of lactose in a few species came up negative. $\endgroup$ – Bryan Hanson Apr 27 '18 at 23:57
  • $\begingroup$ @BryanHanson Thanks. Do you want to make it an answer or should I (you are probably more able to write an answer as you have access to the article)? $\endgroup$ – Remi.b Apr 28 '18 at 0:05
  • $\begingroup$ @Remi.b I'll put it as an answer with nod to you. $\endgroup$ – Bryan Hanson Apr 28 '18 at 0:07
  • $\begingroup$ Is your question as in your title "is there non-milk lactose in Nature?", or is it "did the lactase gene exist in bacteria before mammals appeared?" As you explain in your reasoning. The two are not necessarily the same. $\endgroup$ – David Apr 28 '18 at 16:00
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Preamble

This is an answer to the motivation expressed in the question:

“…whether lactose-utilising enzymes in bacteria evolved only after the appearance of mammals”

rather than that in the title. The presence or absence of lactose elsewhere in Nature will not decide this question.

Answer

The name lactase is applied to the mammalian enzyme that is able to catalyse the hydrolysis of the disaccharide, lactose, to glucose and galactose, allowing the digestion of this milk sugar by the suckling infant. However bacterial and plant enzymes that are normally referred to as β-galactosidases can also catalyse the same reaction.

The role of the β-galactosidases in bacteria such as Lactobacillus lactis that are found in a dairy environment may well be to hydrolyse milk lactose, as may be that of Escherichia coli, found in the human gut. However this is unlikely to be generally true for all bacteria. This is because β-galactosidases are found in a wide spectrum of bacteria, including extremophiles† which cannot possibly have had access to milk lactose.

β-galactosidases are generally members of a group of enzymes termed glycoside hydrolases and have been categorized into a number of families on the basis of sequence similarity. The enzymes in these families differ in their substrate specificity. To explain the existence of such enzymes in bacteria which do not have access to lactase it has been suggested that their natural substrates may be other disaccharides. This is actually the subject of a PhD Thesis by S.A. Shipkowski, who suggests that some β-galactosidases may actually hydrolyse β-1,4 galactobiose:

lactose and galactobiose

The significance of this is that the polysaccharide, pectin, found in certain plant cell walls, could be broken down by by other enzymes in the bacteria, yielding galactobiose.

Conclusion

Although the β-galactosidase in certain bacteria may be optimized to hydrolyse milk lactose, the gene encoding it is almost certainly derived from an ancestral glycoside hydrolases gene, the product of which predated the appearance of mammals and was used to hydrolyse other disaccharides derived from polysaccharides originating from various sources.

† It should be emphasized that the β-galactosidase from extreme thermophiles such as Thermus thermophilus KNOUC114 — isolated from a hot spring — hydrolyses lactose well. Indeed, its thermostablity has attracted interest in its development for use in the food industry (Ahn et al. 2012).

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  • $\begingroup$ The Shipkowski PhD thesis that you linked, it says on the tile page that it was written to fulfill "requirements for the degree of doctor of philosophy"...? What is the relevance of the thesis to philosophy, though? $\endgroup$ – ning Apr 30 '18 at 12:00
  • $\begingroup$ @ning — Natural philosophy provides the ‘Ph’ in PhD or the ‘Phil’ in D.Phil. I would also quote Shakespear here “There are more things in heaven and earth… than are dreamt of in your philosophy.”. $\endgroup$ – David Apr 30 '18 at 12:16
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Here is the abstract from the article that @Remi.b found:

The presence of lactose in Forsythia, Achras zapota and Zizyphus jujuba has been reported previously. We have reexamined its presence using gas-liquid chromatography, paper chromatography and high performance liquid chromatography. Although all three plants contained glucose, fructose, myo-inositol (except A. zapota) and sucrose, lactose was not detected. Hence, the presence of lactose in the plant kingdom should be re-examined.

"Is lactose really present in plants?" Takahiro Toba, Satoshi Nagashima, Susumu Adachi First published: 1991 https://doi.org/10.1002/jsfa.2740540217

So at least this possible source of lactose in plants is disproved.

EDIT: I also searched Web of Science for "lactose AND plant*" in the title field, and there are no articles other than the one @Remi.b found.

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  • $\begingroup$ This isn't really an answer to your question, being merely a refutation of one source. However if you will post it do NOT post the quotation as an image, but take the trouble to type it out. Images of text are not indexable and discriminate against people with sight defects. $\endgroup$ – David Apr 28 '18 at 10:46
  • $\begingroup$ @David Two excellent points, edited accordingly, thank you. $\endgroup$ – Bryan Hanson Apr 28 '18 at 14:02

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