Dear Dr. B:

You wrote:

Hope all ok with you. Could please write me the difference in meaning between these two following sentences:

‘rat renal proximal tubules’

and ‘renal proximal rat tubules’

OR

‘the selective enzyme receptor’
and ‘the enzyme selective receptor’

when there is no hyphen between words. Your kindly cooperation will be appreciated.

Our reply:
Generally, unless there is an understood compound word or a comma between the modifiers, the word nearest the noun modifies the noun. The word next to that modifier modifies both the words, and so on.

For example: “green truck” — “green” modifies “truck.”
“new green truck” — “new” modifies the phrase “green truck.”
“big new green truck” — “big” modifies the phrase “new green truck.”

Now I am not a medical doctor, so I am not certain but in the examples you gave, it sounds like the second example in both cases does not make sense. In the first example I think you are trying to say “renal proximal tubules from rats,” which is what the first phrase means. The second means “proximal rat tubules from kidneys.” I think kidneys have proximal tubules, not rat tubules!

Similarly “selective enzyme receptor” sounds correct. What “selective enzyme receptors” means is “receptors of enzymes that are selective.” The second means “selective receptors belonging to enzymes.” I am no physician, but I believe you mean the first.

I hope this helps, and I hope the example I gave makes this clearer.