No, I don't think so. I just reviewed Chapter 6 of Malt by John Mallett. There are different enzymes for breaking down proteins and starches. DP is a measure of the enzyme content for breaking down starches (i.e. amylases). During the germination process the protein matrix surrounding the starches must be broken down before the starches are exposed and can be broken down by the amylases. Malting stops the germination process before the starches are broken down, while leaving the enzymes to do that. The protein level left over is a function of the starting protein level and exactly how the malting process proceeds. A proficient maltster can apparently play with the ratio of protein to amlyase as desired, so protein level and DP are not related. For less proficient maltsters they are loosely correlated, but it isn't exactly correct to say that DP is a function of protein level.
Actually, that is not quite correct. There is a strong correllation between protein content and diastatic power because a malted barley’s diastatic power is in large part dependent on the amount of beta-amylase found in the barley corns. There is a strong positive correllation between protein content and beta-amylase.
Here is a link to a publication that studied the malting characteristics of five Indian cultivars: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5785396/
From the “Protien content” section:
“Before malting, variety DWRUB52 had the least protein content, while variety DWRB92 had the most. Similarly, among the malted grains, variety DWRUB52 showed the lowest and DWRB92, highest protein content.“
From the “Diastatic power” section:
“The diastatic power of the different cultivars increased significantly after malting. DWRB92 had the highest value both in unmalted (81.05 DP°) and malted (142.45 DP°) grains, while DWRB101 showed lowest diastatic power among all varieties. Different varieties of unmalted grain also showed some diastatic power (58.98–81.05 DP°), which may be due to the presence of free β-amylase enzyme in mature resting grain. β-amylase enzyme is synthesized during grain development and appears in mature grain both in the free and the bound forms (Evans et al. 1997; Yin et al. 2002). Malted barley showed a strong positive correlation between diastatic power and protein content (r = 0.90) (Table 2).”
Now, here is wherw the plot thickens.
From the “Beta-amylase activity” section:
“The β-amylase enzyme activity ranged from 13.97 Betamyl-3 U/g for DWRUB52 to 18.26 Betamyl-3 U/g for DWRB92 in unmalted resting grain, while the corresponding values were 12.55 Betamyl-3 U/g for DWRUB52 and 15.97 Betamyl-3 U/g for DWRB92. Although the cultivar DWRB92 showed highest β-amylase enzyme activity, this was not true for its α-amylase activity. Cultivar DWRUB52, which showed lowest β-amylase activity had higher α-amylase activity than DWRB92. β-amylase is one of the most important components of the diastatic power of malt as there is a strong correlation (r = 0.74) observed between the levels of β-amylase activity in a malt and its diastatic power (Table 2). Very strong correlation (r = 0.85) was also found between protein content of grain and β-amylase activity, which agrees with previous reports by Swanston (1980) and Arends et al. (1995). However, the correlation between β-amylase and α-amylase activities was very weak (r = 0.12).”
From the “Hot water extract” (HWE) section:
“Extractable solid in wort from malt during mashing is known as hot water extract and it is one of the key parameters for selection of good quality malt. The hot water extract percentage of the different barley malt varieties is presented in Fig. 2. DWRUB52 malt had the highest hot water extract value (73.12%) and DWRB92, the lowest (71.82%). Although the correlation (Table 2) between hot water extract and α-amylase activity was very weak (r = 0.03), there was strong to very strong negative correlation between hot water extract and diastatic power (r = − 0.64) and grain protein content (r = − 0.86).”
With the above out of the way, the Bavarian and Bohemian brewmasters who established the American lager brewing industry knew what they were doing when they added corn to their grist, even though doing so violated the beer purity law. It was not as much about reducing production costs as it was about reducing protein levels. While corn has a protein level that is not much lower than that of barley, it is mostly insoluble; therefore, it is left in the mash tun, resulting in a lower combined extractable protein level. Lower extractable protein level resulted in a more stable finished beer. Six-row had a higher diastatic power level than the 2-row found in their home countries, which made the addition of 20+% corn possible. Modern American lager is the result of WWII. American breweries needed to come up with a product that they could sell to women, which resulted in the addition of higher levels of adjuncts to lighten the body and the reduction of hopping rates to reduce bitterness. After the war, most breweries never went back to pre-WWII-style lager. Why? Because they sold more product with a lighter, less hoppy beer. The mega beer sold today is closer to a less full-bodied version of Western lager than it is to Eastern lager. That is because a large percentage of rice is added to the grist. Rice is mostly grown west of the Mississippi, which is why it became the adjunct of choice for brewers west of the the Mississippi.